Display device and electronic device

ABSTRACT

An object of the invention is to provide a circuit technique which enables reduction in power consumption and high definition of a display device. A switch controlled by a start signal is provided to a gate electrode of a transistor, which is connected to a gate electrode of a bootstrap transistor. When the start signal is input, a potential is supplied to the gate electrode of the transistor through the switch, and the transistor is turned off. The transistor is turned off, so that leakage of a charge from the gate electrode of the bootstrap transistor can be prevented. Accordingly, time for storing a charge in the gate electrode of the bootstrap transistor can be shortened, and high-speed operation can be performed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/974,328, filed Aug. 23, 2013, now allowed, which is a continuation ofU.S. application Ser. No. 13/174,895, filed Jul. 1, 2011, now U.S. Pat.No. 8,520,159, which is continuation of U.S. application Ser. No.11/853,215, filed Sep. 11, 2007, now U.S. Pat. No. 7,978,274, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2006-269905 on Sep. 29, 2006, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device including a circuitformed using a transistor. In particular, the present invention relatesto a display device using an electrooptical element such as a liquidcrystal element, a light-emitting element, or the like, and an operationmethod thereof.

2. Description of the Related Art

In recent years, with the increase of large display devices such asliquid crystal televisions, display devices have been activelydeveloped. In particular, a technique for forming a pixel circuit and adriver circuit including a shift register and the like (hereinafter alsoreferred to as an internal circuit) over the same insulating substrateby using transistors formed of a non-crystalline semiconductor(hereinafter also referred to as amorphous silicon) has been activelydeveloped because the technique greatly contributes to reduction inpower consumption and cost. The internal circuit formed over theinsulating substrate is connected to a controller IC or the like(hereinafter also referred to as an external circuit) through an FPC(Flexible Printed Circuit) or the like, and its operation is controlled.

Among the aforementioned internal circuits, a shift register usingtransistors formed of a non-crystalline semiconductor (hereinafter alsoreferred to as amorphous silicon transistors) has been devised. FIG.100A shows a structure of a flip-flop included in a conventional shiftregister (Reference 1: Japanese Published Patent Application No.2004-157508). The flip-flop in FIG. 100A includes a transistor 11 (abootstrap transistor), a transistor 12, a transistor 13, a transistor14, a transistor 15, a transistor 16, and a transistor 17, and isconnected to a signal line 21, a signal line 22, a wiring 23, a signalline 24, a power supply line 25, and a power supply line 26. A startsignal, a reset signal, a clock signal, a power supply potential VDD,and a power supply potential VSS are input to the signal line 21, thesignal line 22, the signal line 24, the power supply line 25, and thepower supply line 26, respectively. An operation period of the flip-flopin FIG. 100A is divided into a set period, a selection period, a resetperiod, and a non-selection period as shown in a timing chart in FIG.100B.

In the set period, an H-level signal is input from the signal line 21and a potential of a node 41 is increased to VDD-Vth15 (Vth15: athreshold voltage of the transistor 15), so that the node 41 is in afloating state while the transistor 11 is kept on. The transistor 16 isin an on state when an H-level signal is input from the signal line 21;and the transistor 16 is turned off when the transistor 14, a gateelectrode of which is connected to the node 41, is turned on and apotential of a node 42 is at L level. That is, a charge is leaked from agate electrode of the transistor 11 during a period from the time whenan H-level signal is input from the signal line 21 until the transistor16 is turned off.

Here, a signal with a potential of VDD is referred to as an H-level anda signal a potential of which is VSS is referred to as an L-levelsignal. L level refers to a state where a potential of the L-levelsignal is VSS.

In display devices in References 2 and 3, a shift register formed ofamorphous silicon transistors is used for a scan line driver circuit,and video signals are input to sub-pixels of R, G, and B from one signalline, so that the number of signal lines is decreased to one third.Thus, in the display devices in References 2 and 3, the number ofconnections between a display panel and a driver IC is reduced(Reference 2: Jin Young Choi, et al., “A Compact and Cost-efficientTFT-LCD through the Triple-Gate Pixel Structure”, SOCIETY FORINFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICALPAPERS, Volume XXXVII, pp. 274-276; and Reference 3: Yong Soon Lee, etal., “Advanced TFT-LCD Data Line Reduction Method”, SOCIETY FORINFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICALPAPERS, Volume XXXVII, pp. 1083-1086).

SUMMARY OF THE INVENTION

According to the related art, the gate electrode of the bootstraptransistor is in a floating state while the bootstrap transistor is kepton. However, in the related art, time is required to make the gateelectrode of the bootstrap transistor in a floating state while thebootstrap transistor is kept on; therefore, there is a problem thathigh-speed operation cannot be performed. Further, when amorphoussilicon is used for a semiconductor layer of a transistor, there is aproblem that a threshold voltage of the transistor is shifted. Inaddition, it has been suggested that the number of signal lines isdecreased to one third and the number of connection points between adisplay panel and a driver IC is reduced (References 2 and 3);practically, the number of connection points of the driver IC isrequired to be further decreased.

That is, a circuit technique for operating a shift register with highspeed and a circuit technique for suppressing variation of a thresholdvoltage of a transistor remain as problems which cannot be solved by therelated art. Further, a technique for reducing the number of connectionpoints of a driver IC mounted on a display panel, reduction in powerconsumption of a display device, and increase in size or definition of adisplay device also remain as problems.

In a display device in this specification, a gate electrode of atransistor, which is connected to a gate electrode of a bootstraptransistor, is provided with a switch controlled by a start signal. Whenthe start signal is input, a potential is supplied to the gate electrodeof the transistor through the switch, and the transistor is turned off.The transistor is turned off, so that leakage of a charge from the gateelectrode of the bootstrap transistor can be prevented. Accordingly,time for storing a charge in the gate electrode of the bootstraptransistor can be shortened, and high-speed operation can be performed.

Note that various types of switches can be used as a switch shown inthis document (a specification, a claim, a drawing, and the like). Anelectrical switch, a mechanical switch, and the like are given asexamples. That is, any element can be used without being limited to aparticular type as long as it can control a current flow. For example, atransistor (e.g., a bipolar transistor or a MOS transistor), a diode(e.g., a PN diode, a PIN diode, a Schottky diode, a MIM (Metal InsulatorMetal) diode, a MIS (Metal Insulator Semiconductor) diode, or adiode-connected transistor), a thyristor, or the like can be used as aswitch. Further, a logic circuit combining such elements can be used asa switch.

In the case where a transistor is used as a switch, polarity (aconductivity type) of the transistor is not particularly limited becauseit operates just as a switch. However, when off-current is preferablysmall, a transistor of polarity with smaller off-current is preferablyused. However, when less off-current is preferable, a transistor ofpolarity with less off-current is preferably used. As a transistor withless off-current, a transistor having an LDD region, a transistor havinga multi-gate structure, and the like are given as examples. Further, ann-channel transistor is preferably used when a potential of a sourceterminal of the transistor operating as a switch is close to a lowpotential side power supply (e.g., Vss, GND, or 0 V). On the other hand,a p-channel transistor is preferably used when the potential of thesource terminal of the transistor operating as a switch is close to ahigh potential side power supply (e.g., Vdd). This is because when thepotential of the source terminal of the n-channel transistor operatingas a switch is close to a low potential side power supply or thepotential of the source terminal of the p-channel transistor operatingas a switch is close to a high potential side power supply, an absolutevalue of a gate-source voltage can be increased; thus, on/off of theswitch can be easily switched. This is also because reduction in outputvoltage does not often occur since the transistor does not often performa source follower operation.

A CMOS switch may also be employed by using both n-channel and p-channeltransistors. A CMOS switch can easily function as a switch becausecurrent can flow when one of the n-channel transistor and the p-channeltransistor is turned on. For example, a voltage can be output asappropriate whether a voltage of an input signal to the switch is highor low. Further, since a voltage amplitude value of a signal for turningon/off a switch can be decreased, power consumption can be reduced.

When a transistor is used as a switch, the switch includes an inputterminal (one of a source terminal and a drain terminal), an outputterminal (the other of the source terminal and the drain terminal), anda terminal (a gate terminal) for controlling electrical conduction. Onthe other hand, when a diode is used as a switch, the switch does nothave a terminal for controlling electrical conduction in some cases.Therefore, when a diode is used as a switch, the number of wirings forcontrolling terminals can be reduced compared with the case where atransistor is used as a switch.

In this specification, when it is explicitly described that A and B areconnected, the case where A and B are electrically connected, the casewhere A and B are functionally connected, and the case where A and B aredirectly connected are included. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, in structuresdisclosed in this specification, another element may be provided in aconnection relationship shown in drawings and texts, without beinglimited to a predetermined connection relationship, for example,connection relationships shown in the drawings and the texts.

For example, when A and B are electrically connected, one or moreelements which enable electrical connection of A and B (e.g., a switch,a transistor, a capacitor, an inductor, a resistor, or a diode) may beprovided between A and B. In addition, when A and B are functionallyconnected, one or more circuits which enable functional connection of Aand B (e.g., a logic circuit such as an inverter, a NAND circuit, or aNOR circuit; a signal converter circuit such as a DA converter circuit,an AD converter circuit, or a gamma correction circuit; a potentiallevel converter circuit such as a power supply circuit (e.g., a voltagestep-up circuit or a voltage step-down circuit) or a level shiftercircuit for changing a potential level of a signal; a voltage source; acurrent source; a switching circuit; or an amplifier circuit which canincrease signal amplitude, the amount of current, or the like, such asan operational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit, a signal generation circuit; amemory circuit; or a control circuit may be provided between A and B.Alternatively, in the case where A and B are directly connected, A and Bmay be directly connected without interposing another element or anothercircuit therebetween.

When it is explicitly described that A and B are directly connected, thecase where A and B are directly connected (i.e., the case where A and Bare connected without interposing another element or another circuittherebetween) and the case where A and B are electrically connected(i.e., the case where A and B are connected by interposing anotherelement or another circuit therebetween) are included.

When it is explicitly described that A and B are electrically connected,the case where A and B are electrically connected (i.e., the case whereA and B are connected by interposing another element or another circuittherebetween), the case where A and B are functionally connected (i.e.,the case where A and B are functionally connected by interposing anothercircuit therebetween), and the case where A and B are directly connected(i.e., the case where A and B are connected without interposing anotherelement or another circuit therebetween) are included. That is, when itis explicitly described that A and B are electrically connected, thedescription is the same as the case where it is explicitly onlydescribed that A and B are connected.

A display element, a display device which is a device including adisplay element, a light-emitting element, and a light-emitting devicewhich is a device including a light-emitting element can employ varioustypes and can include various elements. For example, as a displayelement, a display device, a light-emitting element, and alight-emitting device, a display medium, contrast, luminance,reflectivity, transmittance, or the like of which is changed byelectromagnetic action, such as an EL element (e.g., an organic ELelement, an inorganic EL element, or an EL element including bothorganic and inorganic materials), an electron emitter, a liquid crystalelement, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), a digital micromirror device(DMD), a piezoelectric ceramic display, or a carbon nanotube can beused. Note that display devices using an EL element include an ELdisplay; display devices using an electron emitter include a fieldemission display (FED), an SED-type flat panel display (SED:Surface-conduction Electron-emitter Display), and the like; displaydevices using a liquid crystal element include a liquid crystal display(e.g., a transmissive liquid crystal display, a transflective liquidcrystal display, a reflective liquid crystal display, a direct-viewliquid crystal display, or a projection type liquid crystal display);and display devices using electronic ink include electronic paper.

As a transistor disclosed in this document (the specification, theclaim, the drawing, and the like), various types of transistors can beemployed without being limited to a certain type. For example, a thinfilm transistor (TFT) including a non-single crystalline semiconductorfilm typified by amorphous silicon, polycrystalline silicon,microcrystalline (also referred to as semi-amorphous) silicon, or thelike can be used. The use of the TFT has various advantages. Forexample, since the TFT can be formed at temperature lower than that ofthe case of using single crystalline silicon, reduction in manufacturingcost or increase in size of a manufacturing device can be realized. Atransistor can be formed using a large substrate with increase in sizeof the manufacturing device. Therefore, a large number of displaydevices can be formed at low cost at the same time. Further, sincemanufacturing temperature is low, a substrate having low heat resistancecan be used. Accordingly, a transistor can be formed over alight-transmitting substrate; thus, transmission of light in a displayelement can be controlled by using the transistor formed over thelight-transmitting substrate. Alternatively, since the thickness of thetransistor is thin, part of a film forming the transistor can transmitlight; thus, an aperture ratio can be increased.

The use of a catalyst (e.g., nickel) when polycrystalline silicon isformed enables further improvement in crystallinity and formation of atransistor having excellent electrical characteristics. Thus, a gatedriver circuit (a scan line driver circuit), a source driver circuit (asignal line driver circuit), and a signal processing circuit (e.g., asignal generation circuit, a gamma correction circuit, a DA convertercircuit) can be formed over the same substrate.

The use of a catalyst (e.g., nickel) when microcrystalline silicon isformed enables further improvement in crystallinity and formation of atransistor having excellent electrical characteristics. At this time,crystallinity can be improved by performing only heat treatment withoutusing laser. Thus, a gate driver circuit (a scan line driver circuit)and part of a source driver circuit (e.g., an analog switch) can beformed over the same substrate. Further, when a laser is not used forcrystallization, unevenness of silicon crystallinity can be suppressed.Therefore, an image with high image quality can be displayed.

Note that polycrystalline silicon and microcrystalline silicon can beformed without using a catalyst (e.g., nickel).

A transistor can be formed using a semiconductor substrate, an SOIsubstrate, or the like. In this case, a MOS transistor, a junctiontransistor, a bipolar transistor, or the like can be used as atransistor described in this specification. Therefore, a smalltransistor with few variations in characteristics, sizes, shapes, or thelike, with high current supply capacity can be formed. By using such atransistor, reduction in power consumption or high integration ofcircuits can be realized.

A transistor including a compound semiconductor or an oxidesemiconductor such as zinc oxide (ZnO), amorphous oxide (a-InGaZnO),silicon germanium (SiGe), gallium arsenide (GaAs), indium zinc oxide(IZO), indium tin oxide (ITO), or tin oxide (SnO), or a thin filmtransistor or the like obtained by thinning such a compoundsemiconductor or a oxide semiconductor can be used. Therefore,manufacturing temperature can be lowered and for example, such atransistor can be formed at room temperature. Accordingly, thetransistor can be formed directly on a substrate having low heatresistance, such as a plastic substrate or a film substrate. Note thatsuch a compound semiconductor or an oxide semiconductor can be used fornot only a channel portion of the transistor but also otherapplications. For example, such a compound semiconductor or an oxidesemiconductor can be used as a resistor, a pixel electrode, or atransparent electrode. Further, since such an element can be formed atthe same time as the transistor, cost can be reduced.

A transistor or the like formed by using an inkjet method or a printingmethod can also be used. Accordingly, the transistor can be formed atroom temperature or at a low vacuum, or can be formed over a largesubstrate. In addition, since the transistor can be formed without usinga mask (a reticle), layout of the transistor can be easily changed.Further, since it is not necessary to use a resist, material cost isreduced and the number of steps can be reduced. Furthermore, since afilm is partially formed as appropriate, a material is not wasted andcost can be reduced compared with a manufacturing method in whichetching is performed after the film is formed over the entire surface.

A transistor or the like including an organic semiconductor or a carbonnanotube can also be used. Accordingly, a transistor can be formed usinga substrate which can be bent. Therefore, a device using the transistorincluding the organic semiconductor or the carbon nanotube, or the likecan resist a shock.

In addition, various other transistors can be used.

A transistor can be formed using various types of substrates. The typeof a substrate is not limited to a certain type. For example, a singlecrystalline substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, a stainless steel substrate, asubstrate including a stainless steel foil, or the like can be used as asubstrate. Alternatively, a skin (e.g., epidermis or corium) orhypodermal tissue of an animal such as a human may be used as asubstrate. In addition, the transistor may be formed using onesubstrate, and then, the transistor may be transferred to anothersubstrate. As a substrate to which the transistor is transferred, asingle crystalline substrate, an SOI substrate, a glass substrate, aquartz substrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, a stainless steel substrate, asubstrate including a stainless steel foil, or the like can be used.Alternatively, a skirt (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human may be used as a substrate to which thetransistor is transferred. By using such a substrate, a transistor withexcellent properties or a transistor with low power consumption can beformed, a device with high durability or high heat resistance can beformed, or reduction in weight can be realized.

A structure of a transistor can be various modes without being limitedto a certain structure. For example, a multi-gate structure having twoor more gate electrodes may be used. When the multi-gate structure isused, a structure where a plurality of transistors are connected inseries is provided since channel regions are connected in series. Themulti-gate structure realizes reduction in off-current or improvement inreliability due to improvement in withstand voltage of the transistor.Alternatively, by using the multi-gate structure, drain-source currentdoes not change much even if drain-source voltage changes when thetransistor operates in a saturation region; thus, voltage-currentcharacteristics with a flat slope can be obtained. By utilizing thevoltage-current characteristics with the flat slope, an ideal currentsource circuit or an active load having an extremely high resistancevalue can be realized. Thus, a differential circuit or a current mirrorcircuit having excellent properties can be realized. In addition, astructure where gate electrodes are formed above and below a channel maybe used. By using the structure where gate electrodes are formed aboveand below the channel, a channel region is enlarged, the amount ofcurrent can be increased because the number of channel regions isincreased, or an S-value can be reduced because a depletion layer iseasily formed. When the gate electrodes are formed above and below thechannel, a plurality of transistors are connected in parallel.

Further, a structure where a gate electrode is formed above a channelregion, a structure where a gate electrode is formed below a channelregion, a staggered structure, an inversely staggered structure, astructure where a channel region is divided into a plurality of regions,or a structure where channel regions are connected in parallel or inseries can be employed. In addition, a source electrode or a drainelectrode may overlap with a channel region (or part thereof). By usingthe structure where the source electrode or the drain electrode mayoverlap with the channel region (or part thereof), an unstable operationdue to accumulation of charge in part of the channel region can beprevented. Further, an LDD region may be provided. By providing the LDDregion, off-current can be reduced or the withstand voltage of thetransistor can be increased to improve reliability. Alternatively,drain-source current does not fluctuate much even if drain-sourcevoltage fluctuates when the transistor operates in the saturationregion, so that characteristics where a slope of voltage-currentcharacteristics is flat can be obtained.

Various types of transistors can be used for a transistor in thisspecification and the transistor can be formed using various types ofsubstrates. Accordingly, all of circuits which are necessary to realizea predetermined function may be formed using the same substrate. Forexample, all of the circuits which are necessary to realize thepredetermined function may be formed using a glass substrate, a plasticsubstrate, a single crystalline substrate, an SOI substrate, or anyother substrate. When all of the circuits which are necessary to realizethe predetermined function are formed using the same substrate, thenumber of component parts can be reduced to cut cost and the number ofconnections between circuit components can be reduced to improvereliability. Alternatively, part of the circuits which are necessary torealize the predetermined function may be formed using one substrate andanother part of the circuits which are necessary to realize thepredetermined function may be formed using another substrate. That is,not all of the circuits which are necessary to realize the predeterminedfunction are required to be formed using the same substrate. Forexample, part of the circuits which are necessary to realize thepredetermined function may be formed using transistors over a glasssubstrate and another part of the circuits which are necessary torealize the predetermined function may be formed using a singlecrystalline substrate, so that an IC chip formed by a transistor on thesingle crystalline substrate may be connected to the glass substrate byCOG (Chip On Glass) and the IC chip may be provided over the glasssubstrate. Alternatively, the IC chip may be connected to the glasssubstrate by TAB (Tape Automated Bonding) or a printed wiring board.When part of the circuits are formed using the same substrate in thismanner, the number of the component parts can be reduced to cut cost andthe number of connections between the circuit components can be reducedto improve reliability. In addition, since circuits in a portion withhigh driving voltage or a portion with high driving frequency consumelarge power, the circuits in such portions are formed using a singlecrystalline substrate and using an IC chip formed by the circuit insteadof using the same substrate; thus, increase in power consumption can beprevented.

In this specification, one pixel corresponds to one element brightnessof which can be controlled. For example, one pixel corresponds to onecolor element and brightness is expressed with the one color element.Accordingly, in the case of a color display device having color elementsof R (Red), G (Green), and B (Blue), the minimum unit of an image isformed of three pixels of an R pixel, a G pixel, and a B pixel. Notethat the color elements are not limited to three colors, and colorelements of more than three colors may be used or a color other than RGBmay be added. For example, RGBW may be used by adding W (white). Inaddition, RGB added with one or more colors of yellow, cyan, magentaemerald green, vermilion, and the like may be used. Further, a colorsimilar to at least one of R, and B may be added to RGB. For example, R,B1, and B2 may be used. Although both B1 and B2 are blue, they haveslightly different frequency. Similarly, R1, R2, C; and B may be used.By using such color elements, display which is closer to the real objectcan be performed and power consumption can be reduced. As anotherexample, in the case of controlling brightness of one color element byusing a plurality of regions, one region may correspond to one pixel.For example, in the case of performing area ratio gray scale display orthe case of including a subpixel, a plurality of regions which controlbrightness are provided in each color element and gray scales areexpressed with all of the regions, and one region which controlsbrightness may correspond to one pixel. In that case, one color elementincludes a plurality of pixels. Alternatively, even when the pluralityof the regions which control brightness are provided in one colorelement, these regions may be collected as one pixel. In that case, onecolor element includes one pixel. Further, when brightness is controlledby a plurality of regions in one color element, regions which contributeto display may have different area dimensions depending on pixels. Inthat case, in the plurality of the regions which control brightness inone color element, signals supplied to each region may be slightlyvaried to widen a viewing angle. That is, potentials of pixel electrodesincluded in the plurality of the regions in one color element may bedifferent from each other. Accordingly, voltages applied to liquidcrystal molecules are varied depending on the pixel electrodes.Therefore, the viewing angle can be widened.

Note that when it is explicitly described as one pixel (for threecolors), it corresponds to the case where three pixels of R, G, and Bare considered as one pixel. When it is explicitly described as onepixel (for one color), it corresponds to the case where the plurality ofthe regions are provided in each color element and collectivelyconsidered as one pixel.

In this document, pixels are provided (arranged) in matrix in somecases. Here, description that pixels are provided (arranged) in matrixincludes the case where the pixels are arranged in a straight line andthe case where the pixels are arranged in a jagged line, in alongitudinal direction or a lateral direction. For example, in the caseof performing full color display with three color elements (e.g., RGB),the following cases are included therein: the case where the pixels arearranged in stripes, the case where dots of the three color elements arearranged in a delta pattern, and the case where dots of the three colorelements are provided in Bayer arrangement. Note that the color elementsare not limited to three colors, and color elements of more than threecolors may be employed, for example, RGBW (W corresponds to white), RGBadded with one or more of yellow, cyan, magenta, and the like, or thelike. Further, the size of display regions may be different betweenrespective dots of color elements. Thus, power consumption can bereduced or the life of a light-emitting element can be prolonged.

In this document, an active matrix method in which an active element isincluded in a pixel or a passive matrix method in which an activeelement is not included in a pixel can be used.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also various active elements(non-linear elements), for example, a MIM (Metal Insulator Metal), a TFD(Thin Film Diode), or the like can be used. Since such an element hasfew number of manufacturing steps, manufacturing cost can be reduced ora yield can be improved. Further, since the size of the element issmall, an aperture ratio can be improved, and power consumption can bereduced and high luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcost can be reduced or the yield can be improved. Further, since anactive element (a non-linear element) is not used, the aperture ratiocan be improved, and power consumption can be reduced and high luminancecan be achieved.

A transistor is an element having at least three terminals of a gate, adrain, and a source. The transistor includes a channel region between adrain region and a source region, and current can flow through the drainregion, the channel region, and the source region. Here, since thesource and the drain of the transistor may change depending on astructure, operating conditions, and the like of the transistor, it isdifficult to define which is a source or a drain. Therefore, in thisspecification, a region functioning as a source and a drain is notcalled the source or the drain in some cases. In such a case, one of thesource and the drain may be referred to as a first terminal and theother thereof may be referred to as a second terminal. Alternatively,one of the source and the drain, may be referred to as a first electrodeand the other thereof may be referred to as a second electrode. Furtheralternatively, one of the source and the drain may be referred to as asource region and the other thereof may be referred to as a drainregion.

A transistor may be an element having at least three terminals of abase, an emitter, and a collector. In this case also, one of the emitterand the collector may be referred to as a first terminal and the otherterminal may be referred to as a second terminal.

A gate corresponds to all or part of a gate electrode and a gate wiring(also referred to as a gate line, a gate signal line, a scan line, ascan signal line, or the like). A gate electrode corresponds to aconductive film which overlaps with a semiconductor forming a channelregion with a gate insulating film interposed therebetween. Note thatpart of the gate electrode overlaps with an LDD (Lightly Doped Drain)region, the source region, or the drain region with the gate insulatingfilm interposed therebetween in some cases. A gate wiring corresponds toa wiring for connecting a gate electrode of each transistor to eachother, a wiring for connecting a gate electrode included in each pixelto each other, or a wiring for connecting a gate electrode to anotherwiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which functions as both a gate electrode and a gate wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a gate electrode or a gate wiring. That is, there is aregion where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, in the case where a channelregion overlaps with part of an extended gate wiring, the overlappedportion (the region, the conductive film, the wiring, or the like)functions as both a gate wiring and a gate electrode. Accordingly, sucha portion (a region, a conductive film, a wiring, or the like) may becalled either a gate electrode or a gate wiring.

A portion (a region, a conductive film, a wiring, or the like) which isformed of the same material as a gate electrode and forms the sameisland as the gate electrode to be connected to the gate electrode mayalso be called a gate electrode. Similarly, a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a gate wiring and forms the same island as the gate wiringto be connected to the gate wiring may be called a gate wiring. In astrict sense, such a portion (a region, a conductive film, a wiring, orthe like) does not overlap with a channel region or does not have afunction to connect the gate electrode to another gate electrode in somecases. However, there is a portion (a region, a conductive film, awiring, or the like) which is formed of the same material as a gateelectrode or a gate wiring and forms the same island as the gateelectrode or the gate wiring to be connected to the gate electrode orthe gate wiring. Thus, such a portion (a region, a conductive film, awiring, or the like) may also be called either a gate electrode or agate wiring.

In a multi-gate transistor, for example, a gate electrode of onetransistor is often connected to a gate electrode of another transistorby using a conductive film which is formed of the same material as thegate electrode. Since such a portion (a region, a conductive film, awiring, or the like) is a portion (a region, a conductive film, awiring, or the like) for connecting the gate electrode and another gateelectrode, it may be called a gate wiring, and it may also be called agate electrode since a multi-gate transistor can be considered as onetransistor. That is, a portion (a region, a conductive film, a wiring,or the like) which is formed of the same material as a gate electrode ora gate wiring and forms the same island as the gate electrode or thegate wiring to be connected to the gate electrode or the gate wiring maybe called either a gate electrode or a gate wiring. In addition, part ofa conductive film which connects the gate electrode and the gate wiringand is formed of a material different from that of the gate electrodeand the gate wiring may also be called either a gate electrode or a gatewiring.

A gate terminal corresponds to part of a portion (a region, a conductivefilm, a wiring, or the like) of a gate electrode or a portion (a region,a conductive film, a wiring, or the like) which is electricallyconnected to the gate electrode.

When a gate electrode is called a gate wiring, a gate line, a gatesignal line, a scan line, a scan signal line, or the like, there is thecase where a gate of a transistor is not connected to a wiring. In thiscase, the gate wiring, the gate line, the gate signal line, the scanline, or the scan signal line corresponds to a wiring formed in the samelayer as the gate of the transistor, a wiring formed of the samematerial of the gate of the transistor, or a wiring formed at the sametime as the gate of the transistor in some cases. As examples, a wiringfor storage capacitance, a power supply line, a reference potentialsupply line, and the like can be given.

A source corresponds to all or part of a source region, a sourceelectrode, and a source wiring (also referred to as a source line, asource signal line, a data line, a data signal line, or the like). Asource region corresponds to a semiconductor region containing a largeamount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Accordingly, a regioncontaining a small amount of p-type impurities or n-type impurities,namely, an LDD (Lightly Doped Drain) region is not included in thesource region. A source electrode is part of a conductive layer formedof a material different from that of a source region and electricallyconnected to the source region. However, there is the case where asource electrode and a source region are collectively called a sourceelectrode. A source wiring is a wiring for connecting a source electrodeof each transistor to each other, a wiring for connecting a sourceelectrode of each pixel to each other, or a wiring for connecting asource electrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) functioning as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring. That is, thereis a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, in the case where asource region overlaps with part of an extended source wiring, theoverlapped portion (the region, the conductive film, the wiring, or thelike) functions as both a source wiring and a source electrode.Accordingly, such a portion (a region, a conductive film, a wiring, orthe like) may be called either a source electrode or a source wiring.

A portion (a region, a conductive film, a wiring, or the like) which isformed of the same material as a source electrode and forms the sameisland as the source electrode to be connected to the source electrode,or a portion (a region, a conductive film, a wiring, or the like) whichconnects a source electrode and another source electrode may also becalled a source electrode. Further, a portion which overlaps with asource region may be called a source electrode. Similarly, a regionwhich is formed of the same material as a source wiring and forms thesame island as the source wiring to be connected to the source wiringmay also be called a source wiring. In a strict sense, such a portion (aregion, a conductive film, a wiring, or the like) does not overlap witha channel region or does not have a function to connect the sourceelectrode to another source electrode in some cases. However, there is aportion (a region, a conductive film, a wiring, or the like) which isformed of the same material as a source electrode or a source wiring andforms the same island as the source electrode or the source wiring to beconnected to the source electrode or the source wiring. Thus, such aportion (a region, a conductive film, a wiring, or the like) may also becalled either a source electrode or a source wiring.

For example, part of a conductive film which connects a source electrodeand a source wiring and is formed of a material which is different fromthat of the source electrode or the source wiring may be called either asource electrode or a source wiring.

A source terminal corresponds to part of a source region, a sourceelectrode, or a portion (a region, a conductive film, a wiring, or thelike) which is electrically connected to the source electrode.

When a source electrode is called a source wiring, a source line, asource signal line, a data line, a data signal line, or the like, thereis the case in which a source (a drain) of a transistor is not connectedto a wiring. In this case, the source wiring, the source line, thesource signal line, the data line, or the data signal line correspondsto a wiring formed in the same layer as the source (the drain) of thetransistor, a wiring formed of the same material of the source (thedrain) of the transistor, or a wiring formed at the same time as thesource (the drain) of the transistor in some cases. As examples, awiring for storage capacitance, a power supply line, a referencepotential supply line, and the like can be given.

Note that a drain is similar to the source.

A semiconductor device corresponds to a device having a circuitincluding a semiconductor element (e.g., a transistor, a diode, or athyristor). The semiconductor device may also include all devices whichcan function by utilizing semiconductor characteristics.

A display element corresponds to an optical modulation element, a liquidcrystal element, a light-emitting element, an EL element (an organic ELelement, an inorganic EL element, or an EL element including bothorganic and inorganic materials), an electron-emissive element, anelectrophoresis element, a discharging element, a light-reflectingelement, a light diffraction element, a DMD, or the like. Note that thepresent invention is not limited thereto.

A display device corresponds to a device including a display element.Note that the display device also refers to a display panel itself inwhich a plurality of pixels including display elements are formed overthe same substrate as a peripheral driver circuit for driving thepixels. In addition, the display device may also include a peripheraldriver circuit provided over a substrate by wire bonding or bumpbonding, namely, an IC chip connected by so-called COG, TAB, or thelike. Further, the display device may also include a FPC to which an ICchip, a resistor, a capacitor, an inductor, a transistor, or the like isattached. The display device may also include a printed wiring board(PWB) which is connected through an FPC and to which an IC chip, aresistor, a capacitor, an inductor, a transistor, or the like isattached. The display device may also include an optical sheet such as apolarizing plate or a retardation plate. The display device may alsoinclude a lighting device, a housing, an audio input and output device,a light sensor, or the like. Here, a lighting device such as a backlightunit may include a light guide plate, a prism sheet, a diffusion sheet,a reflective sheet, a light source (e.g., an LED or a cold cathodefluorescent lamp), a cooling device (e.g., a water cooling device or anair cooling device), or the like.

A lighting device corresponds to a device including a backlight unit, alight guide plate, a prism sheet, a diffusion sheet, a reflective sheet,a light source (e.g., an LED, a cold cathode fluorescent lamp, or a hotcathode fluorescent lamp), a cooling device, or the like.

A light-emitting device corresponds to a display device including alight-emitting element.

A reflective device corresponds to a device including a light-reflectingelement, a light diffraction element, a light reflecting electrode, orthe like.

A liquid crystal display device corresponds to a display deviceincluding a liquid crystal element. Liquid crystal display devicesinclude a direct-view liquid crystal display, a projection liquidcrystal display, a transmissive liquid crystal display, a reflectiveliquid crystal display, a transflective liquid crystal display, and thelike.

A driving device corresponds to a device including a semiconductorelement, an electric circuit, or an electronic circuit. For example, atransistor (also referred to as a selection transistor, a switchingtransistor, or the like) which controls input of a signal from a sourcesignal line to a pixel, a transistor which supplies voltage or currentto a pixel electrode, a transistor which supplies voltage or current toa light-emitting element, and the like are examples of the drivingdevice. A circuit (also referred to as a gate driver, a gate line drivercircuit, or the like) which supplies a signal to a gate signal line, acircuit (also referred to as a source driver, a source line drivercircuit, or the like) which supplies a signal to a source signal lineare also examples of the driving device.

A display device, a semiconductor device, a lighting device, a coolingdevice, a light-emitting device, a reflective device, a driving device,and the like overlap with each other in some cases. For example, adisplay device includes a semiconductor device and a light-emittingdevice in some cases. Further, a semiconductor device includes a displaydevice and a driving device in some cases.

In this document, when it is explicitly described that B is formed on Aor that B is formed over A, it does not necessarily mean that B isformed in direct contact with A. The description includes the case whereA and B are not in direct contact with each other, that is, the casewhere another object is interposed between A and B. Here, each of A andB corresponds to an object (e.g., a device, an element, a circuit, awiring, an electrode, a terminal, a conductive film, or a layer).

For example, when it is explicitly described that a layer B is formed on(or over) a layer A, it includes both the case where the layer B isformed in direct contact with the layer A, and the case where anotherlayer (e.g., a layer C or a layer D) is formed in direct contact withthe layer A and the layer B is formed over the layer C or D. Note thatanother layer (e.g., a layer C or a layer D) may be a single layer or aplurality of layers.

Similarly, when it is explicitly described that B is formed above A, itdoes not necessarily mean that B is in direct contact with A, andanother object may be interposed between A and B. For example, when itis explicitly described that a layer B is formed above a layer A, itincludes both the case where the layer B is formed in direct contactwith the layer A, and the case where another layer (e.g., a layer C or alayer D) is formed in direct contact with the layer A and the layer B isformed over the layer C or D. Note that another layer (e.g., a layer Cor a layer D) may be a single layer or a plurality of layers.

When it is explicitly described that B is formed in direct contact withA, it does not include the case where another object is interposedbetween A and B and only includes the case where B is formed in directcontact with A.

Note that the same can be said when it is explicitly described that B isformed below or under A.

With a structure disclosed in this specification, a shift register canoperate with high speed. In particular, even when amorphous silicon isused as a semiconductor layer of a transistor, a shift register canoperate with high speed. Therefore, a semiconductor device such as aliquid crystal display device, to which the shift register is applied,can operate with high speed, and increase in size or high definition ofthe semiconductor device can be easily realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C each show a structure of a flip-flop shown in EmbodimentMode 1.

FIG. 2 is a timing chart showing operations of the flip-flop shown inFIGS. 1A to 1C.

FIGS. 3A to 3D each show an operation of the flip-flop shown in FIGS. 1Ato 1C.

FIGS. 4A to 4C each show a structure of a flip-flop shown in EmbodimentMode 1.

FIGS. 5A and 5B each show a structure of a flip-flop shown in EmbodimentMode 1.

FIG. 6 is a timing chart showing operations of a flip-flop shown inEmbodiment Mode 1.

FIG. 7 shows a structure of a shift register shown in Embodiment Mode 1.

FIG. 8 is a timing chart showing operations of the shift register shownin FIG. 7.

FIG. 9 is a timing chart showing operations of the shift register shownin FIG. 7.

FIG. 10 shows a structure of a shift register shown in Embodiment Mode1.

FIG. 11 shows a structure of a display device shown in Embodiment Mode1.

FIG. 12 is a timing chart showing a writing operation of the displaydevice shown in FIG. 11.

FIG. 13 shows a structure of a display device shown in Embodiment Mode1.

FIG. 14 shows a structure of a display device shown in Embodiment Mode1.

FIG. 15 is a timing chart showing a writing operation of the displaydevice shown in FIG. 14.

FIG. 16 is a timing chart showing operations of a flip-flop shown inEmbodiment Mode 2.

FIG. 17 is a timing chart showing operations of a flip-flop shown inEmbodiment Mode 2.

FIG. 18 shows a structure of a shift register shown in Embodiment Mode2.

FIG. 19 is a timing chart showing operations of the shift register shownin FIG. 18.

FIG. 20 is a timing chart showing operations of the shift register shownin FIG. 18.

FIG. 21 shows a structure of a display device shown in Embodiment Mode2.

FIG. 22 shows a structure of a display device shown in Embodiment Mode2.

FIG. 23 shows a structure of a flip-flop shown in Embodiment Mode 3.

FIG. 24 is a timing chart showing operations of the flip-flop shown inFIG. 23.

FIG. 25 shows a structure of a shift register shown in Embodiment Mode3.

FIG. 26 is a timing chart showing operations of the shift register shownin FIG. 25.

FIG. 27 shows a structure of a flip-flop shown in Embodiment Mode 4.

FIG. 28 is a timing chart showing operations of the flip-flop shown inFIG. 27.

FIG. 29 is a top plan view of the flip-flop shown in FIG. 5A.

FIGS. 30A to 30C each show a structure of a buffer shown in FIG. 10.

FIG. 31 shows a structure of a signal line driver circuit shown inEmbodiment Mode 5.

FIG. 32 is a timing chart showing operations of the signal line drivercircuit shown in FIG. 31.

FIG. 33 shows a structure of a signal line driver circuit shown inEmbodiment Mode 5.

FIG. 34 is a timing chart showing operations of the signal line drivercircuit shown in FIG. 33.

FIG. 35 shows a structure of a signal line driver circuit shown inEmbodiment Mode 5.

FIGS. 36A to 36C each show a structure of a protective diode shown inEmbodiment Mode 6.

FIGS. 37A and 37B each show a structure of a protective diode shown inEmbodiment Mode 6.

FIGS. 38A to 38C each show a structure of a protective diode shown inEmbodiment Mode 6.

FIGS. 39A to 39C each show a structure of a display device shown inEmbodiment Mode 7.

FIGS. 40A to 40G show a process for manufacturing a semiconductor deviceaccording to the invention.

FIG. 41 shows a structure of a semiconductor device according to theinvention.

FIG. 42 shows a structure of a semiconductor device according to theinvention.

FIG. 43 shows a structure of a semiconductor device according to theinvention.

FIG. 44 shows a structure of a semiconductor device according to theinvention.

FIGS. 45A to 45C shows one driving method of a semiconductor deviceaccording to the invention.

FIGS. 46A to 46C shows one driving method of a semiconductor deviceaccording to the invention.

FIGS. 47A to 47C each show a structure of a display device in asemiconductor device according to the invention.

FIGS. 48A and 48B each show a structure of a peripheral circuit in asemiconductor device according to the invention.

FIG. 49 shows a peripheral component of a semiconductor device accordingto the invention.

FIGS. 50A to 50D each show a peripheral component of a semiconductordevice according to the invention.

FIG. 51 shows a peripheral component of a semiconductor device accordingto the invention.

FIGS. 52A to 52C each show a structure of a peripheral circuit in asemiconductor device according to the invention.

FIG. 53 shows a peripheral component of a semiconductor device accordingto the invention.

FIGS. 54A and 54B each show a structure of a panel circuit in asemiconductor device according to the invention.

FIG. 55 shows a structure of a panel circuit in a semiconductor deviceaccording to the invention.

FIG. 56 shows a structure of a panel circuit in a semiconductor deviceaccording to the invention.

FIGS. 57A and 57B are cross-sectional views of display elements in asemiconductor device according to the invention.

FIGS. 58A to 58D are cross-sectional views of display elements in asemiconductor device according to the invention.

FIGS. 59A to 59D are cross-sectional views of display elements in asemiconductor device according to the invention.

FIGS. 60A to 60D are cross-sectional views of display elements in asemiconductor device according to the invention.

FIG. 61 is a top plan view of a pixel in a semiconductor deviceaccording to the invention.

FIGS. 62A and 62B each are top plan views of a pixel in a semiconductordevice according to the invention.

FIGS. 63A and 63B each are top plan views of a pixel in a semiconductordevice according to the invention.

FIG. 64 shows an example of a pixel layout of a semiconductor deviceaccording to the invention.

FIGS. 65A and 65B each show an example of a pixel layout of asemiconductor device according to the invention.

FIGS. 66A and 66B each show an example of a pixel layout of asemiconductor device according to the invention.

FIGS. 67A and 67B show one driving method of a semiconductor deviceaccording to the invention.

FIGS. 68A and 68B show one driving method of a semiconductor deviceaccording to the invention.

FIG. 69 shows a structure of a pixel in a semiconductor device accordingto the invention.

FIG. 70 shows a structure of a pixel in a semiconductor device accordingto the invention.

FIG. 71 shows a structure of a pixel in a semiconductor device accordingto the invention.

FIG. 72A shows an example of a pixel layout of a semiconductor deviceaccording to the invention, and FIG. 72B is a cross-sectional viewthereof.

FIGS. 73A to 73E are cross-sectional views of display elements in asemiconductor device according to the invention.

FIGS. 74A to 74C are cross-sectional views of display elements in asemiconductor device according to the invention.

FIGS. 75A to 75C are cross-sectional views of display elements in asemiconductor device according to the invention.

FIGS. 76A and 76B each show a structure of a semiconductor deviceaccording to the invention.

FIG. 77 shows a structure of a semiconductor device according to theinvention.

FIG. 78 shows a structure of a semiconductor device according to theinvention.

FIG. 79 shows a structure of a semiconductor device according to theinvention.

FIGS. 80A to 80C each show a structure of a semiconductor deviceaccording to the invention.

FIG. 81 shows a structure of a semiconductor device according to theinvention.

FIGS. 82A to 82E each show one driving method of a semiconductor deviceaccording to the invention.

FIGS. 83A and 83B each show one driving method of a semiconductor deviceaccording to the invention.

FIGS. 84A to 84C each show one driving method of a semiconductor deviceaccording to the invention.

FIGS. 85A and 85B each show one driving method of a semiconductor deviceaccording to the invention.

FIG. 86 shows a structure of a semiconductor device according to theinvention.

FIGS. 87A and 87B show electronic devices using a semiconductor deviceaccording to the invention.

FIG. 88 shows a structure of a semiconductor device according to theinvention.

FIGS. 89A to 89C show electronic devices using a semiconductor deviceaccording to the invention.

FIG. 90 shows an electronic device using a semiconductor deviceaccording to the invention.

FIG. 91 shows an electronic device using a semiconductor deviceaccording to the invention.

FIG. 92 shows an electronic device using a semiconductor deviceaccording to the invention.

FIG. 93 shows an electronic device using a semiconductor deviceaccording to the invention.

FIGS. 94A and 94B show electronic devices using a semiconductor deviceaccording to the invention.

FIGS. 95A and 95B show an electronic device using a semiconductor deviceaccording to the invention.

FIGS. 96A to 96C show electronic devices using a semiconductor deviceaccording to the invention.

FIGS. 97A and 97B show electronic devices using a semiconductor deviceaccording to the invention.

FIG. 98 shows an electronic device using a semiconductor deviceaccording to the invention.

FIGS. 99A to 99D each show a structure of the buffer shown in FIG. 10.

FIG. 100A shows a structure of a conventional flip-flop, and FIG. 100Bis a timing chart thereof.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention will be describedwith reference to drawings. However, the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that modes and details can be variously changedwithout departing from the scope and the spirit of the presentinvention. Therefore, the present invention is not construed as beinglimited to description of the embodiment modes. Note that in structureof the embodiment modes described hereinafter, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and repeated description is omitted.

Embodiment Mode 1

In this embodiment mode, structures and driving methods of a flip-flop,a driver circuit including the flip-flop, and a display device includingthe driver circuit are described.

A basic structure of a flip-flop in this embodiment mode is describedwith reference to FIG. 1A. A flip-flop of FIG. 1A includes a firsttransistor 101, a second transistor 102, a third transistor 103, afourth transistor 104, a fifth transistor 105, a sixth transistor 106, aseventh transistor 107, and an eighth transistor 108. In this embodimentmode, the first transistor 101, the second transistor 102, the thirdtransistor 103, the fourth transistor 104, the fifth transistor 105, thesixth transistor 106, the seventh transistor 107, and the eighthtransistor 108 are n-channel transistors and each of them is turned onwhen a gate-source voltage (Vgs) exceeds a threshold voltage (Vth).

In the flip-flop in this embodiment mode, all the first to eighthtransistors 101 to 108 are n-channel transistors. Further, in theflip-flop in this embodiment mode, amorphous silicon can be used as asemiconductor layer of each transistor. Therefore, simplification of amanufacturing process, reduction in manufacturing cost, and improvementin yield can be realized. Even when polysilicon or single crystallinesilicon is used as the semiconductor layer of the transistor,simplification of a manufacturing process can be realized.

Connection relationships of the flip-flop in FIG. 1A are described. Afirst electrode (one of a source electrode and a drain electrode) of thefirst transistor 101 is connected to a fifth wiring 125, and a secondelectrode (the other of the source electrode and the drain electrode) ofthe first transistor 101 is connected to a third wiring 123. A firstelectrode of the second transistor 102 is connected to a fourth wiring124, and a second electrode of the second transistor 102 is connected tothe third wiring 123. A first electrode of the third transistor 103 isconnected to a sixth wiring 126, a second electrode of the thirdtransistor 103 is connected to a gate electrode of the second transistor102, and a gate electrode of the third transistor 103 is connected tothe sixth wiring 126. A first electrode of the fourth transistor 104 isconnected to an eighth wiring 128, a second electrode of the fourthtransistor 104 is connected to the gate electrode of the secondtransistor 102, and a gate electrode of the fourth transistor 104 isconnected to a gate electrode of the first transistor 101. A firstelectrode of the fifth transistor 105 is connected to a seventh wiring127, a second electrode of the fifth transistor 105 is connected to thegate electrode of the first transistor 101, and a gate electrode of thefifth transistor 105 is connected to the first wiring 121. A firstelectrode of the sixth transistor 106 is connected to a tenth wiring130, a second electrode of the sixth transistor 106 is connected to thegate electrode of the first transistor 101, and a gate electrode of thesixth transistor 106 is connected to the gate electrode of the secondtransistor 102. A first electrode of the seventh transistor 107 isconnected to an eleventh wiring 131, a second electrode of the seventhtransistor 107 is connected to the gate electrode of the firsttransistor 101, and a gate electrode of the seventh transistor 107 isconnected to the second wiring 122. A first electrode of the eighthtransistor 108 is connected to a ninth wiring 129, a second electrode ofthe eighth transistor 108 is connected to the gate electrode of thesecond transistor 102, and a gate electrode of the eighth transistor 108is connected to the first wiring 121.

A connection point of the gate electrode of the first transistor 101,the gate electrode of the fourth transistor 104, the second electrode ofthe fifth transistor 105, the second electrode of the sixth transistor106, and the second electrode of the seventh transistor 107 are denotedby a node 141. A connection point of the gate electrode of the secondtransistor 102, the second electrode of the third transistor 103, thesecond electrode of the fourth transistor 104, the gate electrode of thesixth transistor 106, and the second electrode of the eighth transistor108 is denoted by a node 142.

The first wiring 121, the second wiring 122, the third wiring 123, andthe fifth wiring 125 may be referred to as a first signal line, a secondsignal line, a third signal line, and a fourth signal line,respectively. The fourth wiring 124, the sixth wiring 126, the seventhwiring 127, the eighth wiring 128, the ninth wiring 129, the tenthwiring 130, and the eleventh wiring 131 may be referred to as a firstpower supply line, a second power supply line, a third power supplyline, a fourth power supply line, a fifth power supply line, a sixthpower supply line, and a seventh power supply line, respectively.

Next, an operation of the flip-flop in FIG. 1A is described withreference to a timing chart of FIG. 2 and FIGS. 3A to 3D. The timingchart of FIG. 2 is described in which an operation period is dividedinto a set period, a selection period, a reset period, and anon-selection period. Note that a set period, a reset period, and anon-selection period may be collectively called a non-selection periodin some cases.

A potential of V1 is supplied to the sixth wiring 126 and the seventhwiring 127. A potential of V2 is supplied to the fourth wiring 124, theeighth wiring 128, the ninth wiring 129, the tenth wiring 130, and theeleventh wiring 131. Here, V1>V2 is satisfied. A signal with a potentialof V1 is referred to as an H-level signal, and a signal with a potentialof V2 is referred to as an L-level signal.

A signal 221, a signal 225, and a signal 222 shown in FIG. 2 are inputto the first wiring 121, the fifth wiring 125, and the second wiring122, respectively. A signal 223 shown in FIG. 2 is output from the thirdwiring 123. Here, each of the signals 221, 225, 222, and 223 is adigital signal in which a potential of an H-level signal is V1(hereinafter also referred to as H level) and a potential of an L-levelsignal is V2 (hereinafter also referred to as L level). The signals 221,225, 222, and 223 may be referred to as a start signal, a clock signal,a reset signal, and an output signal, respectively.

Note that various signals, potentials, or currents may be input to eachof the first wiring 121, the second wiring 122, and the fourth toeleventh wirings 124 to 131.

In a set period shown in (A) of FIG. 2 and FIG. 3A, the signal 221becomes H level, and the fifth transistor 105 and the eighth transistor108 are turned on. Since the signal 222 is L level, the seventhtransistor 107 is turned off. At this time, a potential (a potential241) of the node 141 becomes V1−Vth105 (Vth105: a threshold voltage ofthe fifth transistor 105), which is a value obtained by subtracting thethreshold voltage of the fifth transistor 105 from a potential of theseventh wiring 127 since the second electrode of the fifth transistor105 functions as a source electrode. Thus, the first transistor 101 andthe fourth transistor 104 are tuned on, and the fifth transistor 105 isturned off. At this time, a potential difference (V1−V2) between apotential (V2) of the eighth wiring 128 and a potential (V1) of thesixth wiring 126 is divided by the third transistor 103, the fourthtransistor 104, and the eighth transistor 108, so that a potential (apotential 242) of the node 142 becomes V2+β (β: a given positivenumber). Note that β<Vth102 (Vth102: a threshold voltage of the secondtransistor 102) and β<Vth106 (Vth106: a threshold voltage of the sixthtransistor 106) are satisfied. Thus, the second transistor 102 and thesixth transistor 106 are turned off. Accordingly, in the set period, thethird wiring 123 is electrically connected to the fifth wiring 125 towhich an L-level signal is input, so that a potential of the thirdwiring 123 becomes V2. Therefore, the L-level signal is output from thethird wiring 123. Further, the node 141 is in a floating state while thepotential is kept at V1−Vth105.

The third transistor 103 and the fourth transistor 104 form an inverterin which the node 141 is an input terminal and the node 142 is an outputterminal. Accordingly, the flip-flop in this embodiment mode may beprovided with a circuit functioning as an inverter between the node 141and the node 142.

In the flip-flop in this embodiment mode, V2 is supplied to the node 142through the eighth transistor 108, and timing when the sixth transistor106 is turned off is advanced. Thus, time when the potential of the node142 becomes V1−Vth105 can be shortened. Accordingly, the flip-flop inthis embodiment mode can operate with high speed and can be applied to alarger display device or a display device with higher definition.

Even when the first electrode of the fifth transistor 105 is connectedto the first wiring 121 as shown in FIG. 4B, the flip-flop in thisembodiment mode can operate the same as in the above-described setperiod. Thus, the seventh wiring 127 is not needed in the flip-flop ofFIG. 4B, so that improvement in yield can be realized. Further,reduction in layout area can be realized in the flip-flop of FIG. 4B.

In order to make the potential of the node 142 V2+β, it is preferablethat a value of a ratio W/L of the channel width W to the channel lengthL of the fourth transistor 104 is at least ten times higher than a valueof W/L of the third transistor 103. Accordingly, the transistor size(W×L) of the fourth transistor 104 is made larger. Consequently, thechannel length L of the third transistor 103 is made larger, andpreferably, twice to three times larger than the channel length L of thefourth transistor 104. Thus, the size of the fourth transistor 104 canbe reduced, and reduction in layout can be realized.

In a selection period shown in (B) of FIG. 2 and FIG. 3B, the signal 221becomes L level, and the fifth transistor 105 and the eighth transistor108 are turned off. Since the signal 222 is kept at L level, the seventhtransistor 107 is kept off. At this time, the potential of the node 141is kept at V1−Vth105. Thus, the first transistor 101 and the fourthtransistor 104 are kept on. Further, as this time, the potential of thenode 142 is kept at V2+β. Thus, the second transistor 102 and the sixthtransistor 106 are kept off. Here, an H-level signal is input to thefifth wiring 125, so that the potential of the third wiring 123 startsto increase. Then, the potential of the node 141 is increased fromV1−Vth105 to V1+Vth101+α (Vth101: a threshold voltage of the firsttransistor 101; and α: a given positive number) by a bootstrapoperation. Thus, a potential of the third wiring 123 becomes V1, whichis equal to that of the fifth wiring 125. Accordingly, in the selectionperiod, the third wiring 123 is electrically connected to the fifthwiring 125 to which the H-level signal is input, so that the potentialof the third wiring 123 becomes V1. Therefore, the H-level signal isoutput from the third wiring 123.

The bootstrap operation is performed by capacitive coupling of parasiticcapacitance between the gate electrode and the second electrode of thefirst transistor 101. As shown in FIG. 1B, by provision of a capacitor151 between the gate electrode and the second electrode of the firsttransistor 101, a stable bootstrap operation can be performed andparasitic capacitance of the first transistor 101 can be reduced. In thecapacitor 151, a gate insulating film may be used as an insulatinglayer, and a gate electrode layer and a wiring layer may be used as aconductive layer. Alternatively, a gate insulating film may be used asthe insulating layer, and a gate electrode layer and a semiconductorlayer to which an impurity is added may be used as the conductive layer.Further alternatively, an interlayer film (an insulating film) may beused as the insulating layer, and a wiring layer and a transparentelectrode layer may be used as the conductive layer. In the capacitor151, when a gate electrode layer and a wiring layer are used as theconductive layer, it is preferable that the gate electrode layer beconnected to the gate electrode of the first transistor 101 and thewiring layer be connected to the second electrode of the firsttransistor 101. When a gate electrode layer and a wiring layer are usedas the conductive layer, it is more preferable that the gate electrodelayer be directly connected to the gate electrode of the firsttransistor 101 and the wiring layer be directly connected to the secondelectrode of the first transistor 101. This is because increase inlayout area of the flip-flop due to provision of the capacitor 151 canbe suppressed.

As shown in FIG. 1C, a transistor 152 may be used as the capacitor 151.When a gate electrode of the transistor 152 is connected to the node 141and first and second electrodes of the transistor 152 are connected tothe third wiring 123, the transistor 152 can function as a capacitorwith a large capacity. Note that the transistor 152 can function as acapacitor even when one of the first and second electrodes is in afloating state.

It is necessary that the first transistor 101 supply an H-level signalto the third wiring 123. Therefore, in order to reduce fall time andrise time of the signal 223, a value of W/L of the first transistor 101is preferably the highest among those of the first to eighth transistors101 to 108.

In the set period, it is necessary that the fifth transistor 105 makethe potential of the node 141 (the gate electrode of the firsttransistor 101) V1−Vth105. Therefore, a value of W/L of the fifthtransistor 105 is preferably ½ to ⅕ times, more preferably ⅓ to ¼ timeshigher than that of the first transistor 101.

In a reset period shown in (C) of FIG. 2 and FIG. 3C, the signal 221 iskept at L level, and the fifth transistor 105 and the eighth transistor108 are kept off. Since the signal 222 is at H level, the seventhtransistor 107 is turned on. The potential of the node 141 at this timebecomes V2 since a potential (V2) of the eleventh wiring 131 is suppliedthrough the seventh transistor 107. Thus, the first transistor 101 andthe fourth transistor 104 are turned off. The potential of the node 142at this time becomes V1−Vth103 (Vth103: a threshold voltage of the thirdtransistor 103), which is a value obtained by subtracting the thresholdvoltage of the third transistor 103 from a potential (V1) of the sixthwiring 126 since the second electrode of the third transistor 103functions as a source electrode. Thus, the second transistor 102 and thesixth transistor 106 are turned on. Accordingly, in the reset period,the third wiring 123 is electrically connected to the fourth wiring 124to which V2 is supplied, so that the potential of the third wiring 123becomes V2. Therefore, an L-level signal is output from the third wiring123.

By delaying timing when the seventh transistor 107 is turned on, falltime of the signal 223 can be reduced. This is because an L-level signalinput to the fifth wiring 125 is supplied to the third wiring 123through the first transistor 101 with a large value of W/L.

When the value of W/L of the seventh transistor 107 is reduced and falltime until the potential of the node 141 becomes V2 is increased, thefall time of the signal 223 can be reduced as well. In this case, thevalue of W/L of the seventh transistor 107 is preferably 1/10 to 1/40times, more preferably 1/20 to 1/30 times higher than that of the firsttransistor 101.

As shown in FIG. 4A, by using a resistor 410 instead of the thirdtransistor 130, the potential of the node 142 can be made V1. Therefore,the second transistor 102 and the sixth transistor 106 can be easilyturned on, and improvement in operation efficiency can be realized.Further, as shown in FIG. 4C, a transistor 402 may be connected inparallel with the third transistor 103.

In a non-selection period shown in (D) of FIG. 2 and FIG. 3D, the signal221 is kept at L level, and the fifth transistor 105 and the eighthtransistor 108 are kept off. Further, since the signal 222 becomes Llevel, the seventh transistor 107 is turned off. At this time, thepotential of the node 142 is kept at V1−Vth103. Thus, the secondtransistor 102 and the sixth transistor 106 are kept on. At this time,the potential of the node 141 is kept at V2 since V2 is supplied throughthe sixth transistor 106. Thus, the first transistor 101 and the fourthtransistor 104 are kept off. Accordingly, in the non-selection period,the third wiring 123 is electrically connected to the fourth wiring 124to which V2 is supplied, so that the potential of the third wiring 123is kept at V2. Therefore, an L-level signal is output from the thirdwiring 123.

By making the potential supplied to the sixth wiring 126 lower than V1,the potential of the node 142 can be lowered, and threshold voltageshifts of the second transistor 102 and the sixth transistor 106 can besuppressed. Therefore, in the flip-flop in this embodiment mode,deterioration in characteristics of the transistor can be suppressedeven when amorphous silicon, in which deterioration in characteristics(a threshold voltage shift) obviously appears, is used as asemiconductor layer of the transistor.

Accordingly, since rise time of the potential of the node 141 can bereduced in the set period, the flip-flop in this embodiment mode canoperate with high speed and can be applied to a larger display device ora display device with higher definition.

Here, functions of the first to eighth transistors 101 to 108 aredescribed. The first transistor 101 has a function to select timing forsupplying the potential of the fifth wiring 125 to the third wiring 123;and a function to increase the potential of the node 141 by a bootstrapoperation, and functions as a bootstrap transistor. The secondtransistor 102 has a function to select timing for supplying a potentialof the fourth wiring 124 to the third wiring 123, and functions as aswitching transistor. The third transistor 103 has a function to dividethe potential of the sixth wiring 126 and the potential of the eighthwiring 128, and functions as an element having a resistance component ora resistor. The fourth transistor 104 has a function to select timingfor supplying the potential of the eighth wiring 128 to the node 142,and functions as a switching transistor. The fifth transistor 105 has afunction to select timing for supplying the potential of the seventhwiring 127 to the node 141, and function as an input transistor. Thesixth transistor 106 has a function to select timing for supplying apotential of the tenth wiring 130 to the node 141, and functions as aswitching transistor. The seventh transistor 107 has a function toselect timing for supplying the potential of the eleventh wiring 131 tothe node 141, and functions as a switching transistor. The eighthtransistor 108 has a function to select timing for supplying a potentialof the ninth wiring 129 to the node 142, and functions as a switchingtransistor.

Note that the first to eighth transistors 101 to 108 are not limited totransistors as long as they have the aforementioned functions. Forexample, as the second transistor 102, the fourth transistor 104, thesixth transistor 106, the seventh transistor 107, and the eighthtransistor 108 each functioning as the switching transistor, a diode, aCMOS analog switch, various logic circuits, or the like may be employedas long as it is an element having a switching function. Further, as thefifth transistor 105 functioning as the input transistor, a PN junctiondiode, a diode-connected transistor, or the like may be employed as longas it has a function to select timing for increasing the potential ofthe node 141 to be turned off.

Arrangement, the number, and the like of the transistors are not limitedto those in FIGS. 1A to 1C as long as an operation similar to FIGS. 1Ato 1C is obtained. In this embodiment mode, as is apparent from FIGS. 3Ato 3D describing the operations of the flip-flop in FIG. 1A, electricalconnections in the set period, the selection period, the reset period,and the non-selection period are performed as shown by solid lines inFIGS. 3A to 3D, respectively. Accordingly, a transistor, an element(e.g., a resistor or a capacitor), a diode, a switch, various logiccircuits, or the like may be added as long as a structure is employed inwhich a transistor or the like is arranged to satisfy the aboveconditions so that a flip-flop can operate.

In addition, driving timing of the flip-flop in this embodiment mode isnot limited to the timing chart of FIG. 2 as long as an operationsimilar to FIGS. 1A to 1C is obtained.

For example, as shown in a timing chart of FIG. 6, a period forinputting H-level signals to the first wiring 121, the second wiring122, and the fifth wiring 125 may be reduced. In FIG. 6, as comparedwith the timing chart of FIG. 2, timing when a signal is switched from Llevel to H level is delayed for a period Ta1, and timing when a signalis switched from H level to L level is advanced for a period Ta2. Thus,in a flip-flop to which the timing chart of FIG. 6 is applied,instantaneous current through each wiring is reduced, so that powersaving, suppression of malfunction, improvement in operation efficiency,and the like can be realized. Further, in the flip-flop to which thetiming chart of FIG. 6 is applied, fall time of a signal output from thethird wiring 123 can be reduced in the reset period. This is becausetiming when the potential of the node 141 becomes L level is delayed for(the period Ta1+the period Ta2), so that an L-level signal input to thefifth wiring 125 is supplied to the third wiring 123 through the firsttransistor 101 with a high current capability (with a large channelwidth). Note that portions common to FIG. 2 are denoted by commonreference numerals, and description thereof is omitted.

It is preferable that a relationship between the period Ta1, the periodTa2, and a period Tb satisfy ((Ta1+Tb)/(Ta1+Ta2+Tb))×100<10 [%]. It ismore preferable that the relation satisfy ((Ta1+Tb)/(Ta1+Ta2+Tb))×100<5[%]. Further, it is preferable to satisfy the period Ta1≈the period Ta2.

The first to eleventh wirings 121 to 131 can be freely connected as longas a flip-flop operates similarly to FIGS. 1A to 1C. For example, asshown in FIG. 5A, the first electrode of the second transistor 102, thefirst electrode of the fourth transistor 104, the first electrode of thesixth transistor 106, the first electrode of the seventh transistor 107,and the first electrode of the eighth transistor 108 may be connected toa sixth wiring 506. Further, the first electrode of the fifth transistor105, and the first electrode and the gate electrode of the thirdtransistor 103 may be connected to a fifth wiring 505. Alternatively, asshown in FIG. 5B, the first electrode and the gate electrode of thethird transistor 103 may be connected to a seventh wiring 507. Here, afirst wiring 501, a second wiring 502, a third wiring 503, and a fourthwiring 504 correspond to the first wiring 121, the second wiring 122,the third wiring 123, and the fifth wiring 125 in FIG. 1A.

In flip-flops of FIGS. 5A and 5B, the number of wirings can be reduced,so that improvement in yield and reduction in layout area can berealized. Further, in the flip-flops of FIGS. 5A and 5B, improvement inreliability and operation efficiency can be realized. In addition, inthe flip-flop of FIG. 5B, a potential supplied to the sixth wiring 506can be lowered, so that threshold voltage shifts of the secondtransistor 102 and the sixth transistor 106 can be suppressed.

FIG. 29 shows an example of a top plan view of the flip-flop shown inFIG. 5A. A conductive layer 2901 includes a portion functioning as thefirst electrode of the first transistor 101, and is connected to thefourth wiring 504 through a wiring 2951. A conductive layer 2902includes a portion functioning as the second electrode of the firsttransistor 101, and is connected to the third wiring 503 through awiring 2952. A conductive layer 2903 includes portions functioning asthe gate electrode of the first transistor 101 and the gate electrode ofthe fourth transistor 104. A conductive layer 2904 includes portionsfunctioning as the first electrode of the second transistor 102, thefirst electrode of the sixth transistor 106, the first electrode of thefourth transistor 104, and the first electrode of the eighth transistor108, and is connected to the sixth wiring 506. A conductive layer 2905includes a portion functioning as the second electrode of the secondtransistor 102, and is connected to the third wiring 503 through awiring 2954. A conductive layer 2906 includes portions functioning asthe gate electrode of the second transistor 102 and the gate electrodeof the sixth transistor 106. A conductive layer 2907 includes a portionfunctioning as the first electrode of the third transistor 103, and isconnected to the fifth wiring 505 through a wiring 2955. A conductivelayer 2908 includes portions functioning as the second electrode of thethird transistor 103 and the second electrode of the fourth transistor104, and is connected to the conductive layer 2906 through a wiring2956. A conductive layer 2909 includes a portion functioning as the gateelectrode of the third transistor 103, and is connected to the fifthwiring 505 through the wiring 2955. A conductive layer 2910 includes aportion functioning as the first electrode of the fifth transistor 105,and is connected to the fifth wiring 505 through a wiring 2959. Aconductive layer 2911 includes portions functioning as the secondelectrode of the fifth transistor 105 and the second electrode of theseventh transistor 107, and is connected to the conductive layer 2903through a wiring 2958. A conductive layer 2912 includes a portionfunctioning as the gate electrode of the fifth transistor 105, and isconnected to the first wiring 501 through a wiring 2960. A conductivelayer 2913 includes a portion functioning as the second electrode of thesixth transistor 106, and is connected to the conductive layer 2903through a wiring 2957. A conductive layer 2914 includes a portionfunctioning as the gate electrode of the seventh transistor 107, and isconnected to the second wiring 502 through a wiring 2962. A conductivelayer 2915 includes a portion functioning as the gate electrode of theeighth transistor 108, and is connected to the conductive layer 2912through a wiring 2961. A conductive layer 2916 includes a portionfunctioning as the second electrode of the eighth transistor 108, and isconnected to the conductive layer 2906 through a wiring 2953.

Here, the width of the wiring 2962 is narrower than that of the wiring2951, 2952, 2953, 2954, 2955, 2956, 2957, 2958, 2959, 2960, or 2961.Alternatively, the length of the wiring 2962 is long. That is, thewiring 2962 has a high resistance value. Accordingly, in the resetperiod, timing when a potential of the conductive layer 2914 becomes Hlevel can be delayed. Thus, timing when the seventh transistor 107 isturned on can be delayed, so that a signal of the third wiring 503 canbecome L level in a shorter period. This is because timing when the node141 becomes L level is delayed, and in this delay period, an L-levelsignal is supplied to the third wiring 503 through the first transistor101.

Note that the wirings 2951, 2952, 2953, 2954, 2955, 2956, 2957, 2958,2959, 2960, 2961, and 2962 are similar to pixel electrodes (alsoreferred to as transparent electrodes or reflective electrodes), andformed using a similar process and material thereto.

The portions functioning as the gate electrode, the first electrode, andthe second electrode of the first transistor 101 are portions where theconductive layers including each electrode overlap with a semiconductorlayer 2981. The portions functioning as the gate electrode, the firstelectrode, and the second electrode of the second transistor 102 areportions where the conductive layers including each electrode overlapwith a semiconductor layer 2982. The portions functioning as the gateelectrode, the first electrode, and the second electrode of the thirdtransistor 103 are portions where the conductive layers including eachelectrode overlap with a semiconductor layer 2983. The portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the fourth transistor 104 are portions where the conductivelayers including each electrode overlap with a semiconductor layer 2984.The portions functioning as the gate electrode, the first electrode, andthe second electrode of the fifth transistor 105 are portions where theconductive layers including each electrode overlap with a semiconductorlayer 2985. The portions functioning as the gate electrode, the firstelectrode, and the second electrode of the sixth transistor 106 areportions where the conductive layers including each electrode overlapwith a semiconductor layer 2986. The portions functioning as the gateelectrode, the first electrode, and the second electrode of the seventhtransistor 107 are portions where the conductive layers including eachelectrode overlap with a semiconductor layer 2987. The portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the eighth transistor 108 are portions where the conductivelayers including each electrode overlap with a semiconductor layer 2988.

Next, a structure and a driving method of a shift register including theaforementioned flip-flop in this embodiment mode are described.

A structure of a shift register in this embodiment mode is describedwith reference to FIG. 7. The shift register in FIG. 7 includes nflip-flops (flip-flops 701_1 to 701 _(—) n).

Connection relationships of the shift register in FIG. 7 are described.In a flip-flop 701 _(—) i in an i-th stage (one of the flip-flops 701_1to 701 _(—) n) of the shift register in FIG. 7, the first wiring 121shown in FIG. 1A is connected to a seventh wiring 717 _(—) i−1. Thesecond wiring 122 shown in FIG. 1A is connected to a seventh wiring 717_(—) i+1. The third wiring 123 shown in FIG. 1A is connected to aseventh wiring 717 _(—) i. The fourth wiring 124, the eighth wiring 128,the ninth wiring 129, the tenth wiring 130, and the eleventh wiring 131shown in FIG. 1A are connected to a fifth wiring 715. The fifth wiring125 shown in FIG. 1A is connected to a second wiring 712 in a flip-flopin an odd-numbered stage, and is connected to a third wiring 713 in aflip-flop in an even-numbered stage. The sixth wiring 126 and theseventh wiring 127 shown in FIG. 1A are connected to a fourth wiring714. In the flip-flop 701_1 in a first stage, the first wiring 121 shownin FIG. 1A is connected to a first wiring 711. In the flip-flop 701 _(—)n in an n-th stage, the second wiring 122 shown in FIG. 1A is connectedto a sixth wiring 716.

The first wiring 711, the second wiring 712, the third wiring 713, andthe sixth wiring 716 may be referred to as a first signal line, a secondsignal line, a third signal line, and a fourth signal line,respectively. The fourth wiring 714 and the fifth wiring 715 may bereferred to as a first power supply line and a second power supply line,respectively.

Next, an operation of a shift register in FIG. 10 is described withreference to timing charts of FIGS. 8 and 9. The timing chart of FIG. 8is divided into a scan period and a retrace period. The scan periodcorresponds to a period from the time when output of a selection signalfrom a seventh wiring 717_1 starts to the time when output of aselection signal from a seventh wiring 717 _(—) n ends. The retraceperiod corresponds to a period from the time when output of theselection signal from the seventh wiring 717 _(—) n ends to the timewhen output of the selection signal from the seventh wiring 717_1starts.

A potential of V1 is supplied to the fourth wiring 714, and a potentialof V2 is supplied to the fifth wiring 715.

Signals 811, 812, 813, and 816 shown in FIG. 8 are input to the firstwiring 711, the second wiring 712, the third wiring 713, and the sixthwiring 716, respectively. Here, each of the signals 811, 812, 813, and816 is a digital signal in which a potential of an H-level signal is V1and a potential of an L-level signal is V2. Further, the signals 811,812, 813, and 816 may be referred to as a start signal, a first clocksignal, a second clock signal (an inverted clock signal), and a resetsignal, respectively.

Note that various signals, potentials, or currents may be input to eachof the first to sixth wirings 711 to 716.

Digital signals 817_1 to 817 _(—) n in each of which a potential of anH-level signal is V1 and a potential of an L-level signal is V2 areoutput from the seventh wirings 717_1 to 717 _(—) n. Note that as shownin FIG. 10, the signals may be output from the seventh wirings 717_1 to717 _(—) n through buffers 1001_1 to 1001 _(—) n, respectively. Theshift register in FIG. 10 can easily operate since an output signal ofthe shift register and a transfer signal of each flip-flop can beseparated.

Examples of the buffers 1001_1 to 1001 _(—) n included in the shiftregister of FIG. 10 are described with reference to FIGS. 99A and 99B.In a buffer 8000 shown in FIG. 99A, inverters 8001 a, 8001 b, and 8001 care connected between wirings 8011 and 8012, so that an inverted signalof a signal input to the wiring 8011 is output from the wiring 8012.Note that the number of inverters connected between the wirings 8011 and8012 is not limited, and for example, when even-numbered inverters areconnected between the wirings 8011 and 8012, signal with the samepolarity as that input to the wiring 8011 are output from the wiring8012. In addition, as shown in a buffer 8100 of FIG. 99B, inverters 8002a, 8002 b, and 8002 c connected in series and inverters 8003 a, 8003 b,and 8003 c connected in series may be connected in parallel. In thebuffer 8100 of FIG. 99B, since variation of deterioration incharacteristics of transistors can be averaged, delay and distortion ofthe signal output from the wiring 8012 can be reduced. Further, outputsof the inverters 8002 a and 8003 a, and outputs of the inverters 8002 band 8003 b may be connected.

In FIG. 99A, it is preferable to satisfy (W of a transistor included inthe inverter 8001 a)<(W of a transistor included in the inverter 8001b)<(W of a transistor included in the inverter 8001 c). This is becausedrive capability of a flip-flop (specifically, a value W/L of thetransistor 101 in FIG. 1A) can be small since W of the transistorincluded in the inverter 8001 a is small; thus, layout area of a shiftregister in the invention can be reduced. Similarly, in FIG. 99B, it ispreferable to satisfy (W of a transistor included in the inverter 8002a)<(W of a transistor included in the inverter 8002 b)<(W of atransistor included in the inverter 8002 c). Similarly, in FIG. 99B, itis preferable to satisfy (W of a transistor included in the inverter8003 a)<(W of a transistor included in the inverter 8003 b)<(W of atransistor included in the inverter 8003 c). Further, it is preferableto satisfy (W of the transistor included in the inverter 8002 a)=(W ofthe transistor included in the inverter 8003 a), (W of the transistorincluded in the inverter 8002 b)=(W of the transistor included in theinverter 8003 b), and (W of the transistor included in the inverter 8002c)=(W of the transistor included in the inverter 8003 c).

The inverters shown in FIGS. 99A and 99B are not particularly limited aslong as they can output an inverted signal of a signal input thereto.For example, as shown in FIG. 99C, an inverter may be formed of a firsttransistor 8201 and a second transistor 8202. A signal is input to afirst wiring 8211, a signal is output from a second wiring 8212, V1 issupplied to a third wiring 8213, and V2 is supplied to a fourth wiring8214. When an H-level signal is input to the first wiring 8211, theinverter of FIG. 99C outputs a potential obtained by dividing V1−V2 bythe first transistor 8201 and the second transistor 8202 ((W/L of thefirst transistor 8201)<(W/L of the second transistor 8202)) from thesecond wiring 8212. Further, when an L-level signal is input to thefirst wiring 8211, the inverter of FIG. 99C outputs V1−Vth8201 (Vth8201:a threshold voltage of the first transistor 8201) from the second wiring8212. The first transistor 8201 may be a PN junction diode or simply aresistor as long as it has a resistance component.

As shown in FIG. 99D, an inverter may be formed of a first transistor8301, a second transistor 8302, a third transistor 8303, and a fourthtransistor 8304. A signal is input to a first wiring 8311, a signal isoutput from a second wiring 8312, V1 is supplied to a third wiring 8313and a fifth wiring 8315, and V2 is supplied to a fourth wiring 8314 anda sixth wiring 8316. When an H-level signal is input to the first wiring8311, the inverter of FIG. 99D outputs V2 from the second wiring 8312.At this time, a potential of a node 8341 is at L level, so that thefirst transistor 8301 is turned off. Further, when an L-level signal isinput to the first wiring 8311, the inverter of FIG. 99D outputs V1 fromthe second wiring 8312. At this time, when the potential of the node8341 becomes V1−Vth8303 (Vth8303: a threshold voltage of the thirdtransistor 8303), the node 8341 is in a floating state. As a result, thepotential of the node 8341 is higher than V1+Vth8301 (Vth8301; athreshold voltage of the first transistor 8301) by a bootstrapoperation, so that the first transistor 8301 is turned on. Further, acapacitor may be provided between a second electrode and a gateelectrode of the first transistor 8301 since the first transistor 8301functions as a bootstrap transistor.

As shown in FIG. 30A, an inverter may be formed of a first transistor8401, a second transistor 8402, a third transistor 8403, and a fourthtransistor 8404. The inverter of FIG. 30A is a two-input inverter, andcan perform a bootstrap operation. A signal is input to a first wiring8411, an inverted signal is input to a second wiring 8412, and a signalis output from a third wiring 8413. V1 is supplied to a fourth wiring8414 and a sixth wiring 8416, and V2 is supplied to a fifth wiring 8415and a seventh wiring 8417. When an L-level signal is input to the firstwiring 8411 and an H-level signal is input to the second wiring 8412,the inverter of FIG. 30A outputs V2 from the third wiring 8413. At thistime, a potential of a node 8441 becomes V2, so that the firsttransistor 8401 is turned off. Further, when an H-level signal is inputto the first wiring 8411 and an L-level signal is input to the secondwiring 8412, the inverter of FIG. 30A outputs V1 from the third wiring8413. At this time, when the potential of the node 8441 becomesV1−Vth8403 (Vth8403: a threshold voltage of the third transistor 8403),the node 8441 is in a floating state. As a result, the potential of thenode 8441 is higher than V1−Vth8401 (Vth8401: a threshold voltage of thefirst transistor 8401) by a bootstrap operation, so that the firsttransistor 8401 is turned on. Further, a capacitor may be providedbetween a second electrode and a gate electrode of the first transistor8401 since the first transistor 8401 functions as a bootstraptransistor. It is preferable that one of the first wiring 8411 and thesecond wiring 8412 be connected to the third wiring 123 in FIG. 1A andthe other thereof be connected to the node 142 in FIG. 1A.

As shown in FIG. 30B, an inverter may be formed of a first transistor8501, a second transistor 8502, and a third transistor 8503. Theinverter of FIG. 30B is a two-input inverter, and can perform abootstrap operation. A signal is input to a first wiring 8511, aninverted signal is input to a second wiring 8512, and a signal is outputfrom a third wiring 8513. V1 is supplied to a fourth wiring 8514 and asixth wiring 8516, and V2 is supplied to a fifth wiring 8515. When anL-level signal is input to the first wiring 8511 and an H-level signalis input to the second wiring 8512, the inverter of FIG. 30B outputs V2from the third wiring 8513. At this time, a potential of a node 8541becomes V2, so that the first transistor 8501 is turned off. Further,when an H-level signal is input to the first wiring 8511 and an L-levelsignal is input to the second wiring 8512, the inverter of FIG. 30Boutputs V1 from the third wiring 8513. At this time, when the potentialof the node 8541 becomes V1−Vth8503 (Vth8503: a threshold voltage of thethird transistor 8503), the node 8541 is in a floating state. As aresult, the potential of the node 8541 is higher than V1+Vth8501(Vth8501: a threshold voltage of the first transistor 8501) by abootstrap operation, so that the first transistor 8501 is turned on.Further, a capacitor may be provided between a second electrode and agate electrode of the first transistor 8501 since the first transistor8501 functions as a bootstrap transistor. It is preferable that one ofthe first wiring 8511 and the second wiring 8512 be connected to thethird wiring 123 in FIG. 1A and the other thereof be connected to thenode 142 in FIG. 1A.

As shown in FIG. 30C, an inverter may be formed of a first transistor8601, a second transistor 8602, a third transistor 8603, and a fourthtransistor 8604. The inverter of FIG. 30C is a two-input inverter, andcan perform a bootstrap operation. A signal is input to a first wiring8611, an inverted signal is input to a second wiring 8612, and a signalis output from a third wiring 8613. V1 is supplied to a fourth wiring8614, and V2 is supplied to a fifth wiring 8615 and a sixth wiring 8616.When an L-level signal is input to the first wiring 8611 and an H-levelsignal is input to the second wiring 8612, the inverter of FIG. 30Coutputs V2 from the third wiring 8613. At this time, a potential of anode 8641 becomes V2, so that the first transistor 8601 is turned off.Further, when an H-level signal is input to the first wiring 8611 and anL-level signal is input to the second wiring 8612, the inverter of FIG.30C outputs V1 from the third wiring 8613. At this time, when thepotential of the node 8641 becomes V1−Vth8603 (Vth8603: a thresholdvoltage of the third transistor 8603), the node 8641 is in a floatingstate. As a result, the potential of the node 8641 is higher thanV1+Vth8601 (Vth8601: a threshold voltage of the first transistor 8601)by a bootstrap operation, so that the first transistor 8601 is turnedon. A capacitor may be provided between a second electrode and a gateelectrode of the first transistor 8601 since the first transistor 8601functions as a bootstrap transistor. It is preferable that one of thefirst wiring 8611 and the second wiring 8612 be connected to the thirdwiring 123 in FIG. 1A and the other thereof be connected to the node 142in FIG. 1A.

In FIG. 7, a signal output from the seventh wiring 717 _(—) i−1 is usedas a start signal of the flip-flop 701 _(—) i, and a signal output fromthe seventh wiring 717 _(—) i+1 is used as a reset signal. A startsignal of the flip-flop 701_1 is input from the first wiring 711. Areset signal of the flip-flop 701 _(—) n is input from the sixth wiring716. Note that as the reset signal of the flip-flop 701 _(—) n, a signaloutput from the seventh wiring 717_1 or a signal output from the seventhwiring 717_2 may be used. Alternatively, a dummy flip-flop may beadditionally provided, and an output signal of the dummy flip-flop maybe used. Thus, the number of wirings and the number of signals can bereduced.

As shown in FIG. 9, for example, when the flip-flop 701 _(—) i enters aselection period, an H-level signal (a selection signal) is output fromthe seventh wiring 717 _(—) i. At this time, the flip-flop 701 _(—) i+1enters a set period. After that, the flip-flop 701 _(—) i enters a resetperiod, and an L-level signal is output from the seventh wiring 717 _(—)i. At this time, the flip-flop 701 _(—) i+1 enters a selection period.After that, the flip-flop 701 _(—) i enters a non-selection period, andan L-level signal is kept being output from the seventh wiring 717 _(—)i. At this time, the flip-flop 701 _(—) i+1 enters a reset period.

Thus, in the shift register of FIG. 7, a selection signal can besequentially output from the seventh wiring 717_1 to the seventh wiring717 _(—) n. That is, in the shift register of FIG. 7, the seventhwirings 717_1 to 717 _(—) n can be scanned.

A shift register to which a flip-flop in this embodiment mode is appliedcan operate with high speed, and thus can be applied to a display devicewith higher definition or a larger display device. Further, in a shiftregister to which a flip-flop in this embodiment mode is applied,simplification of a manufacturing process, reduction in manufacturingcost, and improvement in yield can be realized.

Next, a structure and a driving method of a display device including theaforementioned shift register in this embodiment mode are described.Note that a display device in this embodiment mode includes at least aflip-flop in this embodiment mode.

A structure of a display device in this embodiment mode is describedwith reference to FIG. 11. The display device in FIG. 11 includes asignal line driver circuit 1101, a scan line driver circuit 1102, and apixel portion 1104. The pixel portion 1104 includes a plurality ofsignal lines S1 to Sm provided to extend from the signal line drivercircuit 1101 in a column direction, a plurality of scan lines G1 to Gnprovided to extend from the scan line driver circuit 1102 in a rowdirection, and a plurality of pixels 1103 arranged in matrixcorresponding to the signal lines S1 to Sm and the scan lines G1 to Gn.Each pixel 1103 is connected to the signal line Sj (one of the signallines S1 to Sm) and the scan line Gi (one of the scan lines G1 to Gn).

A shift register in this embodiment mode can be applied to the scan linedriver circuit 1102. It is needless to say that a shift register in thisembodiment mode can be also used for the signal line driver circuit1101.

The scan lines G1 to Gn are connected to the seventh wirings 717_1 to717 _(—) n shown in FIGS. 7 and 10.

The signal line and the scan line may be simply called wirings. Thesignal line driver circuit 1101 and the scan line driver circuit 1102each may be called a driver circuit.

The pixel 1103 at least includes a switching element, a capacitor, and apixel electrode. Note that the pixel 1103 may include a plurality ofswitching elements or a plurality of capacitors. Further, a capacitor isnot always needed. The pixel 1103 may include a transistor operating ina saturation region. The pixel 1103 may include a display element suchas a liquid crystal element or an EL element. As the switching element,a transistor or a PN junction diode can be used. When a transistor isused as the switching element, it preferably operates in a linearregion. Further, when the scan line driver circuit 1102 includes onlyn-channel transistors, an n-channel transistor is preferably used as theswitching element. When the scan line driver circuit 1102 includes onlyp-channel transistors, a p-channel transistor is preferably used as theswitching element.

The scan line driver circuit 1102 and the pixel portion 1104 are formedover an insulating substrate 1105, and the signal line driver circuit1101 is not formed over the insulating substrate 1105. The signal linedriver circuit 1101 is formed on a single crystalline substrate, an SOIsubstrate, or another insulating substrate which is different from theinsulating substrate 1105. The signal line driver circuit 1101 isconnected to the signal lines S1 to Sm through a printed wiring boardsuch as an FPC. Note that the signal line driver circuit 1101 may beformed over the insulating substrate 1105, or a circuit forming part ofthe signal line driver circuit 1101 may be formed over the insulatingsubstrate 1105.

The signal line driver circuit 1101 inputs a voltage or a current as avideo signal to the signal lines S1 to Sm. Note that the video signalmay be an analog signal or a digital signal. Positive and negativepolarity of the video signal may be inverted for each frame (i.e., frameinversion driving), may be inverted for each row (i.e., gate lineinversion driving), may be inverted for each column (i.e., source lineinversion driving), or may be inverted for each row and column (i.e.,dot inversion driving). Further, the video signal may be input to thesignal lines S1 to Sm with dot sequential driving or line sequentialdriving. The signal line driver circuit 1101 may input not only thevideo signal but also a certain voltage such as precharge voltage to thesignal lines S1 to Sm. A certain voltage such as the precharge voltageis preferably input in each frame or in each gate selection period.

The scan line driver circuit 1102 inputs a signal to the scan lines G1to Gn and selects (hereinafter also referred to as scans) the scan linesG1 to Gn sequentially from the first row. Then, the scan line drivercircuit 1102 selects the plurality of pixels 1103 to be connected to theselected scan line. Here, one gate selection period refers to a periodin which one scan line is selected, and a non-selection period refers toa period in which the scan line is not selected. A scan signal refers toa signal output to the scan line from the scan line driver circuit 1102.The maximum value of the scan signal is larger than the maximum value ofthe video signal or the maximum voltage of the signal line, and theminimum value of the scan signal is smaller than the minimum value ofthe video signal or the minimum voltage of the signal line.

When the pixel 1103 is selected, the video signal is input to the pixel1103 from the signal line driver circuit 1101 through the signal line.When the pixel 1103 is not selected, the pixel 1103 maintains the videosignal (a potential corresponding to the video signal) input in theselection period.

Although not shown, a plurality of potentials and a plurality of signalsare supplied to the signal line driver circuit 1101 and the scan linedriver circuit 1102.

Next, an operation of the display device shown in FIG. 11 is describedwith reference to a timing chart of FIG. 12. FIG. 12 shows one frameperiod corresponding to a period for displaying an image for one screen.Although one frame period is not particularly limited, it is preferably1/60 seconds or less so that a person viewing an image does not perceivea flicker.

The timing chart of FIG. 12 shows each timing for selecting the scanline G1 in the first row, the scan line Gi in the i-th row, the scanline Gi+1 in the (i+1)th row, and the scan line Gn in the n-th row.

In FIG. 12, the scan line Gi in the i-th row is selected, for example,and the plurality of pixels 1103 connected to the scan line Gi areselected. Then, a video signal is input to each of the plurality ofpixels 1103 connected to the scan line Gi, and each of the plurality ofpixels 1103 maintains a potential corresponding to the video signal.After that, the scan line Gi in the i-th row is non-selected, the scanline Gi+1 in the (i+1)th row is selected, and the plurality of pixels1103 connected to the scan line Gi+1 are selected. Then, a video signalis input to each of the plurality of pixels 1103 connected to the scanline Gi+1, and each of the plurality of pixels 1103 maintains apotential corresponding to the video signal. Thus, in one frame period,the scan lines G1 to Gn are sequentially selected, and the pixels 1103connected to each scan line are also sequentially selected. A videosignal is input to each of the plurality of pixels 1103 connected toeach scan line, and each of the plurality of pixels 1103 maintains apotential corresponding to the video signal.

A display device which uses a shift register in this embodiment mode asthe scan line driver circuit 1102 can operate with high speed; thus,higher definition or further increase in size of the display device canbe realized. Further, in a display device in this embodiment mode,simplification of a manufacturing process, reduction in manufacturingcost, and improvement in yield can be realized.

In the display device of FIG. 11, the signal line driver circuit 1101requiring high-speed operation is formed over a substrate different fromthat for the scan line driver circuit 1102 and the pixel portion 1104.Therefore, amorphous silicon can be used as semiconductor layers of thetransistors included in the scan line driver circuit 1102 and the pixel1103. As a result, simplification of a manufacturing process, reductionin manufacturing cost, and improvement in yield can be realized.Further, increase in size of a display device in this embodiment modecan be realized. Even when polysilicon or single crystalline silicon isused as the semiconductor layer of the transistor, simplification of amanufacturing process can be realized.

When the signal line driver circuit 1101, the scan line driver circuit1102, and the pixel portion 1104 are formed over the same substrate,polysilicon or single crystalline silicon is preferably used as thesemiconductor layers of the transistors included in the scan line drivercircuit 1102 and the pixel 1103.

The number, arrangement, and the like of the driver circuits are notlimited to those shown in FIG. 11 as long as a pixel can be selected anda video signal can be independently written to each pixel as shown inFIG. 11.

For example, as shown in FIG. 13, the scan lines G1 to Gn may be scannedby a first scan line driver circuit 1302 a and a second scan line drivercircuit 1302 b. The first scan line driver circuit 1302 a and the secondscan line driver circuit 1302 b each have a structure similar to that ofthe scan line driver circuit 1102 in FIG. 11, and scan the scan lines G1to Gn at the same timing. Further, the first scan line driver circuit1302 a and the second scan line driver circuit 1302 b may be called afirst driver circuit and a second driver circuit.

Even if a defect occurs in one of the first scan line driver circuit1302 a and the second scan line driver circuit 1302 b, the scan lines G1to Gn can be scanned by the other of the first scan line driver circuit1302 a and the second scan line driver circuit 1302 b; thus, a displaydevice in FIG. 13 can have redundancy. In the display device in FIG. 13,a load (wiring resistance of the scan lines and parasitic capacitance ofthe scan lines) of the first scan line driver circuit 1302 a and a loadof the second scan line driver circuit 1302 b can be reduced to half ofthose in FIG. 11. Thus, delay and distortion of signals (output signalsof the first scan Line driver circuit 1302 a and the second scan linedriver circuit 1302 b) input to the scan lines G1 to Gn can be reduced.Further, since the loads of the first scan line driver circuit 1302 aand the second scan line driver circuit 1302 b can be reduced in thedisplay device of FIG. 13, the scan lines G1 to Gn can be scanned withhigh speed. Since the scan lines G1 to Gn can be scanned with highspeed, increase in size or definition of a panel can be realized. Notethat portions common to the structure of FIG. 11 are denoted by commonreference numerals, and description thereof is omitted.

As another example, FIG. 14 shows a display device in which a videosignal can be written to a pixel with high speed. In the display deviceof FIG. 14, video signals are input to the pixels 1103 in odd-numberedrows from the signal lines in the odd-numbered columns, and are input tothe pixels 1103 in even-numbered rows from the signal lines in theeven-numbered columns. In the display device of FIG. 14, scan lines inodd-numbered stages among the scan lines G1 to Gn are scanned by a firstscan line driver circuit 1402 a, and scan lines in even-numbered stagesamong the scan lines G1 to Gn are scanned by a second scan line drivercircuit 1402 b. Further, input of a start signal to the first scan linedriver circuit 1402 a is delayed for ¼ period of a clock signal withrespect to a start signal input to the second scan line driver circuit1402 b.

The display device of FIG. 14 can perform dot inversion driving simplyby inputting a positive video signal and a negative video signal to thesignal lines in each column in one frame period. Further, the displaydevice of FIG. 14 can perform frame inversion driving by invertingpolarity of the video signal input to each signal line in every oneframe period.

An operation of the display device in FIG. 14 is described withreference to a timing chart of FIG. 15. The timing chart of FIG. 15shows each tinting for selecting the scan line G1 in the first row, thescan line Gi−1 in the (i−1)th row, the scan line Gi in the i-th row, thescan line Gi+1 in the (i+1)th row, and the scan line Gn in the n-th row.Further, in the timing chart of FIG. 15, one selection period is dividedinto a selection period a and a selection period b. The case where thedisplay device in FIG. 14 performs dot inversion driving and frameinversion driving is described with reference to the timing chart ofFIG. 15.

In FIG. 15, the selection period a of the scan line Gi in the i-th row,for example, overlaps with the selection period b of the scan line Gi−1in the (i−1)th row. The selection period b of the scan line Gi in thei-th row overlaps with the selection period a of the scan line Gi+1 inthe (i+1)th row. Therefore, in the selection period a, a video signalsimilar to that input to the pixel 1103 in the (i−1)th row and (j+1)thcolumn is input to the pixel 1103 in the i-th row and j-th column.Further, in the selection period b, a video signal similar to that inputto the pixel 1103 in the i-th row and j-th column is input to the pixel1103 in the (i+1)th row and (j+1)th column. Note that a video signalinput to the pixel 1103 in the selection period b is an original videosignal, and a video signal input to the pixel 1103 in the selectionperiod a is a video signal for precharging the pixel 1103. Accordingly,in the selection period a, each pixel 1103 is precharged by the videosignal input to the pixel 1103 in the (i−1)th row and (j+1)th column,and in the selection period b, an original video signal (in the i-th rowand j-th column) is input to each pixel 1103.

Accordingly, since the video signal can be written to the pixel 1103with high speed, increase in size and definition of the display devicein FIG. 14 can be realized. Further, in the display device of FIG. 14,since the video signals with the same polarity are input to respectivesignal lines in one frame period, the amount of charging and dischargingof each signal line is decreased, and reduction in power consumption canbe realized. Further, since a load of an IC for inputting the videosignal can be greatly decreased in display device of FIG. 14, heatgeneration, power consumption, and the like of the IC can be reduced.Furthermore, since driving frequency of the first scan line drivercircuit 1402 a and the second scan line driver circuit 1402 b in thedisplay device of FIG. 14 can be decreased to approximately half, powersaving can be realized.

In the display device in this embodiment mode, various driving methodscan be performed depending on a structure and a driving method of thepixel 1103. For example, in one frame period, a scan line driver circuitmay scan the scan lines a plurality of times.

A wiring or the like may be added to the display devices in FIGS. 11,13, and 14 depending on a structure of the pixel 1103. For example, apower supply line maintained at a constant potential, a capacitor line,another scan line, or the like may be added. When another scan line isadded, a scan line driver circuit to which a shift register in thisembodiment mode is applied may be added as well. As another example, apixel portion may be provided with a dummy scan line, signal line, powersupply line, or capacitor line.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be applied to or combined with the contents (or part of thecontents) described in another drawing. Further, much more drawings canbe formed by combining each part with another part in theabove-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to or combined with the contents(or part of the contents) described in a drawing in another embodimentmode. Further, much more drawings can be formed by combining each partin each drawing in this embodiment mode with part of another embodimentmode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents described in other embodiment modes, an example of related partthereof, or the like. Therefore, the contents described in otherembodiment modes can be applied to or combined with this embodimentmode.

Embodiment Mode 2

In this embodiment mode, structures and driving methods of a flip-flopdifferent from those in Embodiment Mode 1, a driver circuit includingthe flip-flop, and a display device including the driver circuit aredescribed. Note that portions common to Embodiment Mode 1 are denoted bycommon reference numerals, and detailed description of the same portionsand portions having similar functions is omitted.

As a structure of a flip-flop in this embodiment mode, a structuresimilar to that of the flip-flop in Embodiment Mode 1 can be used. Thus,in this embodiment mode, description of the structure of the flip-flopis omitted. Note that timing for driving the flip-flop is different fromthat in Embodiment Mode 1.

The case where driving timing in this embodiment mode is applied to FIG.1A is described. Note that the driving timing in this embodiment modecan be freely combined with each flip-flop in FIGS. 1B, 1C, 4A to 4C,5A, and 5B as well. Further, the driving timing in this embodiment modecan be freely combined with the driving timing in Embodiment Mode 1 aswell.

An operation of the flip-flop in this embodiment mode is described withreference to the flip-flop in FIG. 1A and a timing chart of FIG. 16. Thetiming chart of FIG. 16 is described in which an operation period isdivided into a set period, a selection period, a reset period, and anon-selection period. Note that the set period is divided into a firstset period and a second set period, and the selection period is dividedinto a first selection period and a second selection period.

A signal 1621, a signal 1625, and a signal 1622 in FIG. 16 are input tothe first wiring 121, the fifth wiring 125, and the second wiring 122,respectively. A signal 1623 in FIG. 16 is output from the third wiring123. Here, the signals 1621, 1625, 1622, and 1623 correspond to thesignals 221, 225, 222, and 223 in FIG. 2, respectively. The signals1621, 1625, 1622, and 1623 may be referred to as a start signal, a clocksignal, a reset signal, and an output signal, respectively.

The flip-flop in this embodiment mode basically operates similarly tothe flip-flop in Embodiment Mode 1. The flip-flop in this embodimentmode is different from the flip-flop in Embodiment Mode 1 in that timingwhen an H-level signal is input to the first wiring 121 is delayed for ¼period of a clock signal.

In the first set period (A1), the second set period (A2), the resetperiod (C), and the non-selection period (D) shown in FIG. 16, theflip-flop in this embodiment mode operates similarly in thenon-selection period (D), the set period (A), the reset period (C), andthe non-selection period (D) shown in FIG. 2, and description thereof isomitted.

As shown in FIG. 17, in the flip-flop in this embodiment mode, thetiming when the H-level signal is input to the second wiring 122 isdelayed for ¼ period of the clock signal, so that fall time of an outputsignal can be significantly decreased. That is, in the flip-flop in thisembodiment mode to which FIG. 17 is applied, an L-level signal is inputto the fifth wiring 125, and the potential of the node 141 is decreasedto approximately V1+Vth101 in a first reset period shown in FIG. 17.Thus, the first transistor 101 is kept on, and an L-level signal isoutput from the third wiring 123. An L-level signal is input to thethird wiring 123 through the first transistor 101 with the large valueof W/L. Therefore, time for the potential of the third wiring 123 tochange from H level to L level can be significantly reduced. After that,in the flip-flop in this embodiment mode, to which FIG. 17 is applied,the seventh transistor 107 is turned on, and the potential of the node141 becomes V2 in a second reset period (C2) in FIG. 17. The potential(a potential 1642) of the node 142 at this time becomes V1−Vth103, andthe third transistor 103 is turned on; thus, an L-level signal is outputfrom the third Wiring 123.

The flip-flop in this embodiment mode can obtain advantageous effectssimilar to those of the flip-flop in Embodiment Mode 1.

Next, a structure and a driving method of a shift register including theaforementioned flip-flop in this embodiment mode are described.

A structure of a shift register in this embodiment mode is describedwith reference to FIG. 18. The shift register in FIG. 18 includes nflip-flops (flip-flops 1801_1 to 1801 _(—) n).

Connection relationships of the shift register in FIG. 18 are described.In a flip-flop 1801 _(—) i in an i-th stage (one of the flip-flops1801_1 to 1801 _(—) n) of the shift register in FIG. 18, the firstwiring 121 shown in FIG. 1A is connected to a tenth wiring 1820 _(—)i−1. The second wiring 122 shown in FIG. 1A is connected to a tenthwiring 1820 _(—) i+2. The third wiring 123 shown in FIG. 1A is connectedto a tenth wiring 1820 _(—) i. The fourth wiring 124, the eighth wiring128, the ninth wiring 129, the tenth wiring 130, and the eleventh wiring131 shown in FIG. 1A are connected to a seventh wiring 1817. The fifthwiring 125 shown in FIG. 1A is connected to a second wiring 1812 in aflip-flop in a (4N−3)th stage (N is a natural number of 1 or more), to athird wiring 1813 in a flip-flop in a (4N−2)th stage, to a fourth wiring1814 in a flip-flop in a (4N−1)th stage, and to a fifth wiring 1815 in aflip-flop in a 4N-th stage. The sixth wiring 126 and the seventh wiring127 shown in FIG. 1A are connected to a sixth wiring 1816. Note that inthe flip-flop 1801_1 in a first stage, the first wiring 121 shown inFIG. 1A is connected to a first wiring 1811. In the flip-flop 1801 _(—)n−1 in an (n−1)th stage, the second wiring 122 shown in FIG. 1A isconnected to a ninth wiring 1819. In the flip-flop 1801 _(—) n in ann-th stage, the second wiring 122 shown in FIG. 1A is connected to aneighth wiring 1818.

When the timing chart of FIG. 17 is applied to the flip-flop in thisembodiment mode, in the flip-flop 1801 _(—) i in the i-th stage, thesecond wiring 122 in FIG. 1A is connected to a tenth wiring 1820 _(—)+3.Accordingly, in the flip-flop 1801 _(—) n−3 in an (n−3)th stage, anadditional wiring is connected to the second wiring 122 in FIG. 1A.

The first wiring 1811, the second wiring 1812, the third wiring 1813,the fourth wiring 1814, the fifth wiring 1815, the eighth wiring 1818,and the ninth wiring 1819 may be referred to as a first signal line, asecond signal line, a third signal line, a fourth signal line, a fifthsignal line, a sixth signal line, and a seven signal line, respectively.The sixth wiring 1816 and the seventh wiring 1817 may be referred to asa first power supply line and a second power supply line, respectively.

Next, an operation of the shift register in FIG. 18 is described withreference to timing charts of FIGS. 19 and 20. Here, the timing chart ofFIG. 19 is divided into a scan period and a retrace period.

The potential of V1 is supplied to the sixth wiring 1816, and thepotential of V2 is supplied to the seventh wiring 1817.

Signals 1911, 1912, 1913, 1914, 1915, 1918, and 1919 shown in FIG. 19are input to the first wiring 1811, the second wiring 1812, the thirdwiring 1813, the fourth wiring 1814, the fifth wiring 1815, the eighthwiring 1818, and the ninth wiring 1819, respectively. Here, each of thesignals 1911, 1912, 1913, 1914, 1915, 1918, and 1919 is a digital signalin which a potential of an H-level signal is V1 and a potential of anL-level signal is V2. Further, the signals 1911, 1912, 1913, 1914, 1915,1918, and 1919 may be referred to as a start signal, a first clocksignal, a second clock signal, a third clock signal, a fourth clocksignal, a first reset signal, and a second reset signal, respectively.

Note that various signals, potentials, or currents may be input to eachof the first to ninth wirings 1811 to 1819.

Digital signals 1920_1 to 1920_n in each of which a potential of anH-level signal is V1 and a potential of an L-level signal is V2 areoutput from the tenth wirings 1820_1 to 1820_n. Similarly to EmbodimentMode 1, the tenth wirings 1820_1 to 1820_n are connected to respectivebuffers, so that the shift register can easily operate.

A signal output from the tenth wiring 1820 _(—) i−1 is used as a startsignal of the flip-flop 1801 _(—) i, and a signal output from the tenthwiring 1820 _(—) i+2 is used as a reset signal. Here, a start signal ofthe flip-flop 1801_1 is input from the first wiring 1811. A second resetsignal of the flip-flop 1801 _(—) n−1 is input from the ninth wiring1819. A first reset signal of the flip-flop 1801 _(—) n is input fromthe eighth wiring 1818. Note that a signal output from the tenth wiring1820_1 may be used as the second reset signal of the flip-flop 1801 _(—)n−1, or a signal output from the tenth wiring 1820_2 may be used as thefirst reset signal of the flip-flop 1801 _(—) n. Alternatively, thesignal output from the tenth wiring 1820_2 may be used as the secondreset signal of the flip-flop 1801 _(—) n−1, and a signal output fromthe tenth wiring 1820_3 may be used as the first reset signal of theflip-flop 1801 _(—) n. Further alternatively, first and second dummyflip-flops may be additionally provided, and output signals of the firstand second dummy flip-flops may be used as the first and second resetsignals. Thus, the number of wirings and the number of signals can bereduced.

As shown in FIG. 20, for example, when the flip-flop 1801 _(—) i entersa first selection period, an H-level signal (a selection signal) isoutput from the tenth wiring 1820 _(—) i. At this time, the flip-flop1801 _(—) i+1 enters' a second set period. Then, even after theflip-flop 1801 _(—) i enters a second selection period, the H-levelsignal is kept being output from the tenth wiring 1820 _(—) i. At thistime, the flip-flop 1801_41 enters a first selection period. After that,when the flip-flop 1801 _(—) i enters a reset period, an L-level signalis output from the tenth wiring 1820 _(—) i. At this time, the flip-flop1801 _(—) i+1 enters a second selection period. Then, even after theflip-flop 1801 _(—) i enters a non-selection period, the L-level signalis kept being output from the tenth wiring 1820 _(—) i. At this time,the flip-flop 1801_ii−1 enters a reset period. Thus, the shift registerof FIG. 18 can output a selection signal sequentially from the tenthwiring 1820_1 to the tenth wiring 1820 _(—) n. Further, in the shiftregister of FIG. 18, the second selection period of the flip-flop 1801_(—) i and the first selection period of the flip-flop 1801 _(—) i+1 arethe same period; thus, the selection signal can be output from the tenthwiring 1820 _(—) i and the tenth wiring 1820 _(—) i+1 in the sameperiod.

A shift register to which a flip-flop in this embodiment mode is appliedcan be applied to a display device with high definition or a largedisplay device. Further, a shift register in this embodiment mode canobtain advantageous effects similar to those of the shift register inEmbodiment Mode 1.

Next, a structure and a driving method of a display device including theaforementioned shift register in this embodiment mode are described.Note that a display device in this embodiment mode includes at least aflip-flop in this embodiment mode.

A structure of a display device in this embodiment mode is describedwith reference to FIG. 21. In the display device of FIG. 21, the scanlines G1 to Gn are scanned by a scan line driver circuit 2102. Further,in the display device of FIG. 21, video signals are input to the pixels1103 in the odd-numbered rows from the signal lines in the odd-numberedrows, and are input to the pixels 1103 in the even-numbered rows fromthe signal lines in the even-numbered rows. Note that portions common tothe structure of FIG. 11 are denoted by common reference numerals, anddescription thereof is omitted.

When a shift register in this embodiment mode is applied to the scanline driver circuit 2102, the display device of FIG. 21 can operatesimilarly to the display device of FIG. 14 by one scan line drivercircuit. Thus, advantageous effects similar to those of the displaydevice of FIG. 14 can be obtained.

Similarly to FIG. 13, the scan lines G1 to Gn may be scanned by a firstscan line driver circuit 2202 a and a second scan line driver circuit2202 b. Thus, advantageous effects similar to those of the displaydevice of FIG. 13 can be obtained. FIG. 22 shows a structure in thiscase.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be applied to or combined with the contents (or part of thecontents) described in another drawing. Further, much more drawings canbe formed by combining each part with another part in theabove-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to or combined with the contents(or part of the contents) described in a drawing in another embodimentmode. Further, much more drawings can be formed by combining each partin each drawing in this embodiment mode with part of another embodimentmode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents described in other embodiment modes, an example of related partthereof, or the like. Therefore, the contents described in otherembodiment modes can be applied to or combined with this embodimentmode.

Embodiment Mode 3

In this embodiment mode, structures and driving methods of a flip-flopdifferent from those in Embodiment Modes 1 and 2, a driver circuitincluding the flip-flop, and a display device including the drivercircuit are described. In a flip-flop in this embodiment mode, an outputsignal and a transfer signal of the flip-flop are output from differentwirings by different transistors. Note that portions common toEmbodiment Modes 1 and 2 are denoted by common reference numerals, anddetailed description of the same portions and portions having similarfunctions is omitted.

A basic structure of a flip-flop in this embodiment mode is describedwith reference to FIG. 23. The flip-flop in FIG. 23 is similar to theflip-flop in FIG. 1A to which a ninth transistor 109 and a tenthtransistor 110 are added.

Connection relationships of the flip-flop in FIG. 23 are described. Afirst electrode of the ninth transistor 109 is connected to a thirteenthwiring 133, a second electrode of the ninth transistor 109 is connectedto a twelfth wiring 132, and a gate electrode of the ninth transistor109 is connected to the node 141. A first electrode of the tenthtransistor 110 is connected to a fourteenth wiring 134, a secondelectrode of the tenth transistor 110 is connected to the twelfth wiring132, and a gate electrode of the tenth transistor 110 is connected tothe node 142. Other connection relationships are similar to FIG. 1A.

The thirteenth wiring 133 and the fourteenth wiring 134 may be referredto as a fifth signal line and an eighth power supply line, respectively.

Next, an operation of the flip-flop in FIG. 23 is described with atiming chart of FIG. 24. Here, the timing chart of FIG. 24 is describedin which an operation period is divided into a set period, a selectionperiod, a reset period, and a non-selection period. Note that a setperiod, a reset period, and a non-selection period may be collectivelycalled a non-selection period in some cases.

The signal 223 and a signal 232 are output from the third wiring 123 andthe twelfth wiring 132, respectively. The signal 232 is an output signalof the flip-flop, and the signal 223 is a transfer signal of theflip-flop. Note that the signal 223 may be used as the output signal ofthe flip-flop, and the signal 232 may be used as the transfer signal ofthe flip-flop.

When the signal 232 is used as the output signal of the flip-flop, andthe signal 223 is used as the transfer signal of the flip-flop, a valueof W/L of the ninth transistor 109 is preferably the highest among thoseof the first to tenth transistors 101 to 110. When the signal 223 is beused as the output signal of the flip-flop, and the signal 232 is beused as the transfer signal of the flip-flop, the value of W/L of thefirst transistor 101 is preferably the highest among those of the firstto tenth transistors 101 to 110.

As has been described above, in this embodiment mode, the output signaland the transfer signal of the flip-flop are output from differentwirings by different transistors. That is, in the flip-flop of FIG. 23,a signal is output from the third wiring 123 by the first transistor 101and the second transistor 102. Further, a signal is output from thetwelfth wiring 132 by the ninth transistor 109 and the tenth transistor110. The ninth transistor 109 and the tenth transistor 110 are connectedin the same manner as the first transistor 101 and the second transistor102; thus, as shown in FIG. 24, the signal (the signal 232) output fromthe twelfth wiring 132 has approximately the same waveform as the signal(the signal 223) output from the third wiring 123.

The first transistor 101 is acceptable as long as it can supply chargesto the gate electrode of the eighth transistor 108 and the gateelectrode of the fifth transistor 105 in the next stage; thus, the valueof the W/L of the first transistor 101 is preferably twice or less thevalue of the W/L of the fifth transistor 105. More preferably, the valueof the W/L of the first transistor 101 is equal to or less than thevalue of the W/L of the fifth transistor 105.

The ninth transistor 109 and the tenth transistor 110 have functionssimilar to those of the first transistor 101 and the second transistor102, respectively. Further, the ninth transistor 109 and the tenthtransistor 110 may be referred to as a buffer portion.

As described above, even when a large load is connected to the twelfthwiring 132 and delay, distortion, or the like of the signal 232 occurs,malfunction of the flip-flop in FIG. 23 can be prevented. This isbecause delay, distortion, or the like of the output signal does notaffect the flip-flop in FIG. 23 since the output signal and the transfersignal of the flip-flop are output from different wirings by differenttransistors.

The flip-flop in FIG. 23 can obtain advantageous effects similar tothose of the flip-flops in Embodiment Modes 1 and 2.

A flip-flop in this embodiment mode can be freely combined with any ofFIGS. 1B, 1C, 4A to 4C, 5A, and 5B. Further, a flip-flop in thisembodiment mode can be combined with each driving timing in EmbodimentModes 1 and 2.

Next, a structure and a driving method of a shift register including theaforementioned flip-flop in this embodiment mode are described.

A structure of a shift register in this embodiment mode is describedwith reference to FIG. 25. The shift register in FIG. 25 includes nflip-flops (flip-flops 2501_1 to 2501 _(—) n).

The flip-flops 2501_1 to 2501 _(—) n, a first wiring 2511, a secondwiring 2512, a third wiring 2513, a fourth wiring 2514, a fifth wiring2515, and a sixth wiring 2516 correspond to the flip-flops 701_1 to 701_(—) n, the first wiring 711, the second wiring 712, the third wiring713, the fourth wiring 714, the fifth wiring 715, and the sixth wiring716 in FIG. 7; and a similar signal or a similar power supply voltagethereto is supplied. Seventh wirings 2517_1 to 2517 _(—) n and eighthwirings 2518_1 to 2518 _(—) n correspond to the seventh wirings 717_1 to717 _(—) n in FIG. 7.

Next, an operation of the shift register in FIG. 25 is described withreference to a timing chart of FIG. 26.

The operation of the shift register in FIG. 25 is different from that ofthe shift register in FIG. 7 in that the output signal and the transfersignal are output to different wirings. Specifically, the output signalsare output to the eighth wirings 2518_1 to 2518 _(—) n, and the transfersignals are output to the seventh wirings 2517_1 to 2517 _(—) n.

Even when a large load (e.g., resistance or capacitance) is connected tothe eighth wirings 2518_1 to 2518 _(—) n, the shift register in FIG. 25can operate without being affected by the load. Further, even when anyof the eighth wirings 2518_1 to 2518 _(—) n is short-circuited with thepower supply line or the signal line, the shift register in FIG. 25 cancontinue to operate normally. Accordingly, in the shift register in FIG.25, improvement in operation efficiency, reliability, and yield can berealized. This is because the transfer signal and the output signal ofeach flip-flop are divided in the shift register in FIG. 25.

A shift register to which a flip-flop in this embodiment mode is appliedcan obtain advantageous effects similar to those of the shift registersin Embodiment Modes 1 and 2.

A shift register in this embodiment mode can be combined with each shiftregister in FIGS. 7 and 10. Further, a shift register in this embodimentmode can be combined with the description in Embodiment Modes 1 and 2.

As a display device in this embodiment mode, each display device inFIGS. 11, 13, 14, 21, and 22 can be used. Thus, a display device in thisembodiment mode can obtain advantageous effects similar to those of thedisplay devices in Embodiment Modes 1 and 2.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be applied to or combined with the contents (or part of thecontents) described in another drawing. Further, much more drawings canbe formed by combining each part with another part in theabove-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to or combined with the contents(or part of the contents) described in a thawing in another embodimentmode. Further, much more drawings can be formed by combining each partin each drawing in this embodiment mode with part of another embodimentmode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents described in other embodiment modes, an example of related partthereof, or the like. Therefore, the contents described in otherembodiment modes can be applied to or combined with this embodimentmode.

Embodiment Mode 4

In this embodiment mode, the case where a p-channel transistor isapplied to a transistor included in a flip-flop in this specification isdescribed. In addition, structures and driving methods of a drivercircuit including the flip-flop and a display device including thedriver circuit are described.

As a flip-flop in this embodiment, the case where a p-channel transistoris used as each transistor included in the flip-flop of FIG. 1A isdescribed. Thus, a flip-flop in FIG. 27 can obtain advantageous effectssimilar to those of the flip-flop in FIG. 1A. Note that a p-channeltransistor may be used as each transistor included in each flip-flopshown in FIGS. 1B, 1C, 4A to 4C, 5A, 5B, and 23. Note also that aflip-flop in this embodiment mode can be freely combined with thedescription in Embodiment Modes 1 to 3.

A basic structure of a flip-flop in this embodiment mode is describedwith reference to FIG. 27. The flip-flop in FIG. 27 includes first toeighth transistors 2701 to 2708. The first to eighth transistors 2701 to2708 correspond to the first to eighth transistors 101 to 108 in FIG.1A. Note that the first to eighth transistors 2701 to 2708 are p-channeltransistors, and each of them is turned on when an absolute value of agate-source voltage (|Vgs|) exceeds an absolute value of a thresholdvoltage (|Vth|), that is, when Vgs becomes lower than Vth.

In the flip-flop in this embodiment mode, the first to eighthtransistors 2701 to 2708 are p-channel transistors. Thus, simplificationof a manufacturing process, reduction in manufacturing cost, andimprovement in yield can be realized in the flip-flop in this embodimentmode.

Connection relationships of the flip-flop in FIG. 27 are similar tothose in FIG. 1A, and description thereof is omitted.

First to eleventh wirings 2721 to 2731 in FIG. 27 correspond to thefirst to eleventh wirings 121 to 131.

Next, an operation of the flip-flop in FIG. 27 is described withreference to a timing chart of FIG. 28. Here, the timing chart of FIG.28 is described in which an operation period is divided into a setperiod, a selection period, a reset period, and a non-selection period.Note that a set period, a reset period, and a non-selection period maybe collectively called a non-selection period in some cases.

The timing chart of FIG. 28 is similar to the timing chart in which Hlevel and L level are reversed in FIG. 2. That is, the flip-flop in FIG.27 is different from the flip-flop in FIG. 1A only in that H level and Llevel of an input signal and an output signal are reversed. Note thatsignals 2821, 2825, 2841, 2842, 2822, and 2823 correspond to the signals221, 225, 241, 242, 222, and 223 in FIG. 2.

Note that V1 and V2 of power supply voltage supplied to the flip-flop inFIG. 27 are reversed to those of the flip-flop of FIG. 1A.

First, an operation of the flip-flop in the set period denoted by (A) ofFIG. 28 is described. A potential 2841 of a node 2741 becomesV2+|Vth2705|. Then, the node 2741 enters a floating state while thepotential is kept at V2+|Vth2705|. At this time, a potential 2842 of anode 2742 becomes V1−θ (θ: a given positive integer). Note that sincethe first transistor 2701 and the second transistor 2702 are turned on,an H-level signal is output from the third wiring 2723.

An operation of the flip-flop in the selection period denoted by (B) ofFIG. 28 is described. The potential 2841 of the node 2741 becomesV2−|Vth2701|−γ (Vth2701: a threshold voltage of the first transistor2701; and γ: a given positive integer). Thus, the first transistor 2701is turned on, and an L-level signal is output from the third wiring2723.

An operation of the flip-flop in the reset period denoted by (C) of FIG.28 is described. The seventh transistor 2707 is turned on, so that thepotential 2841 of the node 2741 becomes V1. Thus, the fust transistor2701 is turned off. At this time, the potential 2842 of the node 2742becomes V2+|Vth2703|, and the second transistor 2702 is turned on. Thus,an H-level signal is output from the third wiring 2723.

An operation of the flip-flop in the non-selection period denoted by (D)of FIG. 28 is described. The potential 2841 of the node 2741 is kept atV1. The potential 2842 of the node 2742 is kept at V2+|Vth2703|, and thesecond transistor 2702 is kept on. Thus, an H-level signal is outputfrom the third wiring 2723.

In a shift register in this embodiment mode, the flip-flop in thisembodiment mode can be combined with each shift register in EmbodimentModes 1 to 3. For example, in the shift register in this embodimentmode, the flip-flop in this embodiment mode can be combined with eachshift register in FIGS. 7, 10, and 25. Note that in the shift registerin this embodiment mode, H level and L level are reversed to those ofeach shift register in Embodiment Modes 1 to 3.

In a display device in this embodiment mode, the shift register in thisembodiment mode can be combined with each display device in EmbodimentModes 1 to 3. For example, the display device in this embodiment modecan be combined with any of the display devices in FIGS. 11, 13, 14, 21,and 22. Note that in display device in this embodiment mode, H level andL level are reversed to those of each display device in Embodiment Modes1 to 3.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be applied to or combined with the contents (or part of thecontents) described in another drawing. Further, much more drawings canbe formed by combining each part with another part in theabove-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to or combined with the contents(or part of the contents) described in a drawing in another embodimentmode. Further, much more drawings can be formed by combining each partin each drawing in this embodiment mode with part of another embodimentmode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents described in other embodiment modes, an example of related partthereof, or the like. Therefore, the contents described in otherembodiment modes can be applied to or combined with this embodimentmode.

Embodiment Mode 5

In this embodiment mode, a signal line driver circuit included in eachdisplay device shown in Embodiment Modes 1 to 4 is described.

A signal line driver circuit in FIG. 31 is described. The signal linedriver circuit in FIG. 56 includes a driver IC 5601, switch groups5602_1 to 5602_M, a first wiring 5611, a second wiring 5612, a thirdwiring 5613, and wirings 5621_1 to 5621_M. Each of the switch groups5602_1 to 5602_M includes a first switch 5603 a, a second switch 5603 b,and a third switch 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, the third wiring 5613, and each oneof the wirings 5621_1 to 5621_M corresponding to the switch groups5602_1 to 5602_M, respectively. Each of the wirings 5621_1 to 5621_M isconnected to three signal lines through the first switch 5603 a, thesecond switch 5603 b, and the third switch 5603 c. For example, thewiring 56213 in the J-th column (one of the wirings 5621_1 to 5621_M) isconnected to a signal line Sj−1, a signal line Sj, and a signal lineSj+1 through the first switch 5603 a, the second switch 5603 b, and thethird switch 5603 c included in the switch group 5602_J.

A signal is input to each of the first wiring 5611, the second wiring5612, and the third wiring 5613.

The driver IC 5601 is preferably formed using a single crystallinesubstrate or a glass substrate using a polycrystalline semiconductor.The switch groups 5602_1 to 5602_M are preferably formed over the samesubstrate as the pixel portion shown in Embodiment Mode 1. Therefore,the driver IC 5601 and the switch groups 5602_1 to 5602_M are preferablyconnected through an FPC or the like.

Next, an operation of the signal line driver circuit in FIG. 31 isdescribed with reference to a timing chart of FIG. 32. The timing chartof FIG. 32 shows the case where a scan line Gi in the i-th row isselected. A selection period of the scan line Gi in the i-th row isdivided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. Note that the signalline driver circuit in FIG. 31 operates similarly to FIG. 32 even when ascan line in another row is selected.

The timing chart of FIG. 32 shows the case where the wiring 5621_J inthe J-th column is connected to the signal line Sj−1, the signal lineSj, and the signal line Sj+1 through the first switch 5603 a, the secondswitch 5603 b, and the third switch 5603 c.

The timing chart of FIG. 32 shows timing when the scan line Gi in thei-th row is selected, timing 5703 a of on/off of the first switch 5603a, timing 5703 b of on/off of the second switch 5603 b, timing 5703 c ofon/off of the third switch 5603 c, and a signal 5721_J input to thewiring 5621_J in the J-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621_1 to 5621_M. For example, a video signal inputto the wiring 5621_J in the first sub-selection period T1 is input tothe signal line Sj−1, a video signal input to the wiring 5621_J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621_J in the third sub-selectionperiod T3 is input to the signal line Sj+1. In the first sub-selectionperiod T1, the second sub-selection period T2, and the thirdsub-selection period T3, the video signals input to the wiring 5621_Jare denoted by Dataj−1, Dataj, and Dataj+1.

As shown in FIG. 32, in the first sub-selection period T1, the firstswitch 5603 a is turned on, and the second switch 5603 b and the thirdswitch 5603 c are turned off. At this time, Dataj−1 input to the wiring5621_J is input to the signal line Sj−1 through the first switch 5603 a.In the second sub-selection period T2, the second switch 5603 b isturned on, and the first switch 5603 a and the third switch 5603 c areturned off. At this time, Dataj input to the wiring 5621_J is input tothe signal line Sj through the second switch 5603 b. In the thirdsub-selection period T3, the third switch 5603 c is turned on, and thefirst switch 5603 a and the second switch 5603 b are turned off. At thistime, Dataj+1 input to the wiring 5621_J is input to the signal lineSj+1 through the third switch 5603 c.

As described above, in the signal line driver circuit of FIG. 31, onegate selection period is divided into three; thus, video signals can beinput to three signal lines from one wiring 5621 in one gate selectionperiod. Therefore, in the signal line driver circuit in FIG. 31, thenumber of connections in which the substrate provided with the driver IC5601 and the substrate provided with the pixel portion are connected canbe approximately one third of the number of signal lines. The number ofconnections is reduced to approximately one third of the number ofsignal lines; therefore, reliability, yield, and the like of the signalline driver circuit in FIG. 31 can be improved.

By applying the signal line driver circuit in this embodiment mode toeach display device shown in Embodiment Modes 1 to 4, the number ofconnections in which the substrate provided with the pixel portion andan external substrate are connected can be further reduced. Therefore,reliability and yield of the display device in the invention can beimproved.

Next, the case where n-channel transistors are used for the first switch5603 a, the second switch 5603 b, and the third switch 5603 c isdescribed with reference to FIG. 33. Note that portions similar to FIG.31 are denoted by the same reference numerals, and detailed descriptionof the same portions and portions having similar functions is omitted.

A first transistor 5903 a in FIG. 33 corresponds to the first switch5603 a in FIG. 31. A second transistor 5903 b in FIG. 33 corresponds tothe second switch 5603 b in FIG. 31. A third transistor 5903 c in FIG.33 corresponds to the third switch 5603 c in FIG. 31.

For example, in the case of the switch group 56023, a first electrode ofthe first transistor 5903 a is connected to the wiring 5621_J, a secondelectrode of the first transistor 5903 a is connected to the signal lineSj−1, and a gate electrode of the first transistor 5903 a is connectedto the first wiring 5611. A first electrode of the second transistor5903 b is connected to the wiring 5621_J, a second electrode of thesecond transistor 5903 b is connected to the signal line Sj, and a gateelectrode of the second transistor 5903 b is connected to the secondwiring 5612. A first electrode of the third transistor 5903 c isconnected to the wiring 5621_J, a second electrode of the thirdtransistor 5903 c is connected to the signal line Sj+1, and a gateelectrode of the third transistor 5903 c is connected to the thirdwiring 5613.

The first transistor 5903 a, the second transistor 5903 b, and the thirdtransistor 5903 c each function as a switching transistor. Further, eachof the first transistor 5903 a, the second transistor 5903 b, and thethird transistor 5903 c is turned on when a signal input to each gateelectrode is at H level, and is turned off when a signal input to eachgate electrode is at L level.

When n-channel transistors are used for the first switch 5603 a, thesecond switch 5603 b, and the third switch 5603 c, amorphous silicon canbe used for a semiconductor layer of a transistor; thus, simplificationof a manufacturing process, reduction in manufacturing cost, andimprovement in yield can be realized. Further, a semiconductor devicesuch as a large display panel can be formed. Even when polysilicon orsingle crystalline silicon is used for the semiconductor layer of thetransistor, simplification of a manufacturing process can also berealized.

In the signal line driver circuit in FIG. 33, n-channel transistors areused for the first transistor 5903 a, the second transistor 5903 b, andthe third transistor 5903 c; however, p-channel transistors may be usedfor the first transistor 5903 a, the second transistor 5903 b, and thethird transistor 5903 c. In the latter case, each transistor is turnedon when a signal input to the gate electrode is at L level, and isturned off when a signal input to the gate electrode is at H level.

Note that arrangement, the number, a driving method, and the like of aswitch are not limited as long as one gate selection period is dividedinto a plurality of sub-selection periods and video signals are input toa plurality of signal lines from one wiring in each of the plurality ofsub-selection periods as shown in FIG. 31.

For example, when video signals are input to three or more signal linesfrom one wiring in each of three or more sub-selection periods, a switchand a wiring for controlling the switch may be added. Note that when oneselection period is divided into four or more sub-selection periods, onesub-selection period becomes too short. Therefore, one selection periodis preferably divided into two or three sub-selection periods.

As another example, as shown in a timing chart of FIG. 34, one selectionperiod may be divided into a precharge period Tp, the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3. The to timing chart of FIG. 34 showstiming when the scan line Gi in the i-th row is selected, timing 5803 aof on/off of the first switch 5603 a, timing 5803 b of on/off of thesecond switch 5603 b, timing 5803 c of on/off of the third switch 5603c, and a signal 5821_J input to the wiring 5621_J in the J-th column. Asshown in FIG. 34, the first switch 5603 a, the second switch 5603 b, andthe third switch 5603 c are tuned on in the precharge period Tp. At thistime, a precharge voltage Vp input to the wiring 5621_) is input to eachof the signal line Sj−1, the signal line Sj, and the signal line Sj+1through the first switch 5603 a, the second switch 5603 b, and the thirdswitch 5603 c. In the first sub-selection period T1, the first switch5603 a is turned on, and the second switch 5603 b and the third switch5603 c are turned off. At this time, Dataj−1 input to the wiring 5621_Jis input to the signal line Sj−1 through the first switch 5603 a. In thesecond sub-selection period T2, the second switch 5603 b is turned on,and the first switch 5603 a and the third switch 5603 c are turned off.At this time, Dataj input to the wiring 5621_) is input to the signalline Sj through the second switch 5603 b. In the third sub-selectionperiod T3, the third switch 5603 c is turned on, and the first switch5603 a and the second switch 5603 b are turned off. At this time,Dataj+1 input to the wiring 5621_J is input to the signal line Sj+1through the third switch 5603 c.

As described above, in the signal line driver circuit of FIG. 31, towhich the timing chart of FIG. 34 is applied, since a prechargeselection period is provided before a sub-selection period, a signalline can be precharged; thus, a video signal can be written to a pixelwith high speed. Note that portions similar to FIG. 32 are denoted bythe same reference numerals, and detailed description of the sameportions and portions having similar functions is omitted.

Also in FIG. 35, one gate selection period can be divided into aplurality of sub-selection periods and video signals can be input to aplurality of signal lines from one wiring in each of the plurality ofsub-selection periods as shown in FIG. 31. Note that FIG. 35 shows onlya switch group 6022_J in the J-th column in a signal line drivercircuit. The switch group 6022_J includes a first transistor 6001, asecond transistor 6002, a third transistor 6003, a fourth transistor6004, a fifth transistor 6005, and a sixth transistor 6006. The firsttransistor 6001, the second transistor 6002, the third transistor 6003,the fourth transistor 6004, the fifth transistor 6005, and the sixthtransistor 6006 are n-channel transistors. The switch group 6022_J isconnected to a first wiring 6011, a second wiring 6012, a third wiring6013, a fourth wiring 6014, a fifth wiring 6015, a sixth wiring 6016,the wiring 5621_J, the signal line Sj−1, the signal line Sj, and thesignal line Sj+1.

A first electrode of the first transistor 6001 is connected to thewiring 5621_J, a second electrode of the first transistor 6001 isconnected to the signal line Sj−1, and a gate electrode of the firsttransistor 6001 is connected to the first wiring 6011. A first electrodeof the second transistor 6002 is connected to the wiring 5621_J, asecond electrode of the second transistor 6002 is connected to thesignal line Sj−1, and a gate electrode of the second transistor 6002 isconnected to the second wiring 6012. A first electrode of the thirdtransistor 6003 is connected to the wiring 5621_J, a second electrode ofthe third transistor 6003 is connected to the signal line Sj, and a gateelectrode of the third transistor 6003 is connected to the third wiring6013. A first electrode of the fourth transistor 6004 is connected tothe wiring 5621_J, a second electrode of the fourth transistor 6004 isconnected to the signal line Sj, and a gate electrode of the fourthtransistor 6004 is connected to the fourth wiring 6014. A firstelectrode of the fifth transistor 6005 is connected to the wiring5621_J, a second electrode of the fifth transistor 6005 is connected tothe signal line Sj+1, and a gate electrode of the fifth transistor 6005is connected to the fifth wiring 6015. A first electrode of the sixthtransistor 6006 is connected to the wiring 5621_J, a second electrode ofthe sixth transistor 6006 is connected to the signal line Sj+1, and agate electrode of the sixth transistor 6006 is connected to the sixthwiring 6016.

The first transistor 6001, the second transistor 6002, the thirdtransistor 6003, the fourth transistor 6004, the fifth transistor 6005,and the sixth transistor 6006 each function as a switching transistor.Further, each of first transistor 6001, the second transistor 6002, thethird transistor 6003, the fourth transistor 6004, the fifth transistor6005, and the sixth transistor 6006 is turned on when a signal input toeach gate electrode is at H level, and is turned off when a signal inputto each gate electrode is at L level.

The first wiring 6011 and the second wiring 6012 in FIG. 35 correspondto a first wiring 5611 in FIG. 33. The third wiring 6013 and the fourthwiring 6014 in FIG. 35 correspond to a second wiring 5612 in FIG. 33.The fifth wiring 6015 and the sixth wiring 6016 in FIG. 35 correspond toa third wiring 5613 in FIG. 33. Note that the first transistor 6001 andthe second transistor 6002 in FIG. 35 correspond to the first transistor5903 a in FIG. 33. The third transistor 6003 and the fourth transistor6004 in FIG. 35 correspond to the second transistor 5903 b in FIG. 33.The fifth transistor 6005 and the sixth transistor 6006 in FIG. 35correspond to the third transistor 5903 c in FIG. 33.

In FIG. 35, in the first sub-selection period T1 shown in FIG. 32, oneof the first transistor 6001 and the second transistor 6002 is turnedon. In the second sub-selection period T2, one of the third transistor6003 and the fourth transistor 6004 is turned on. In the thirdsub-selection period T3, one of the fifth transistor 6005 and the sixthtransistor 6006 is turned on. Further, in the precharge period Tp shownin FIG. 34, either the first transistor 6001, the third transistor 6003,and the fifth transistor 6005; or the second transistor 6002, the fourthtransistor 6004, and the sixth transistor 6006 are turned on.

Thus, in FIG. 35, since the on time of each transistor can be reduced,deterioration in characteristics of the transistor can be suppressed.This is because in the first sub-selection period T1 shown in FIG. 32,for example, the video signal can be input to the signal line Sj−1 whenone of the first transistor 6001 and the second transistor 6002 isturned on. Here, in the first sub-selection period T1 shown in FIG. 32,for example, when both the first transistor 6001 and the secondtransistor 6002 are turned on at the same time, the video signal can beinput to the signal line Sj−1 with high speed.

Two transistors are connected in parallel between the wiring 5621 andthe signal line in FIG. 35; however, the invention is not limitedthereto, and three or more transistors may be connected in parallelbetween the wiring 5621 and the signal line. Thus, deterioration incharacteristics of each transistor can be further suppressed.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be applied to or combined with the contents (or part of thecontents) described in another drawing. Further, much more drawings canbe formed by combining each part with another part in theabove-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to or combined with the contents(or part of the contents) described in a drawing in another embodimentmode. Further, much more drawings can be formed by combining each partin each drawing in this embodiment mode with part of another embodimentmode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents described in other embodiment modes, an example of related partthereof, or the like. Therefore, the contents described in otherembodiment modes can be applied to or combined with this embodimentmode.

Embodiment Mode 6

In this embodiment mode, a structure for preventing a defect due toelectrostatic discharge damage in the display device shown in EmbodimentModes 1 to 4 is described.

Electrostatic discharge damage refers to instant discharge through aninput/output terminal of a semiconductor device when positive ornegative charges stored in the human body or the object touch thesemiconductor device, and damage caused by supplying a large currentflowing within the semiconductor device.

FIG. 36A shows a structure for preventing electrostatic discharge damagecaused in a scan line by a protective diode. FIG. 36A shows a structurewhere the protective diode is provided between a wiring 6111 and thescan line. Although not shown, a plurality of pixels are connected tothe scan line Gi in the i-th row. A transistor 6101 is used as theprotective diode. The transistor 6101 is an n-channel transistor;however, a p-channel transistor may be used, and polarity of thetransistor 6101 may be the same as that of a transistor included in ascan line driver circuit or a pixel.

One protective diode is arranged here; however, a plurality ofprotective diodes may be arranged in series, in parallel, or inseries-parallel.

A first electrode of the transistor 6101 is connected to the scan lineGi in the i-th row, a second electrode of the transistor 6101 isconnected to the wiring 6111, and a gate electrode of the transistor6101 is connected to the scan line Gi in the i-th row.

An operation of FIG. 36A is described. A certain potential is input tothe wiring 6111, which is lower than L level of a signal input to thescan line Gi in the i-th row. When positive or negative charges are notdischarged to the scan line Gi in the i-th row, a potential of the scanline Gi in the i-th row is at H level or L level, so that the transistor6101 is off. On the other hand, when negative charges are discharged tothe scan line Gi in the i-th row, the potential of the scan line Gi inthe i-th row decreases instantaneously. At this time, the potential ofthe scan line Gi in the i-th row is lower than a value obtained bysubtracting a threshold voltage of the transistor 6101 from a potentialof the wiring 6111, so that the transistor 6101 is turned on. Thus, acurrent flows to the wiring 6111 through the transistor 6101. Therefore,the structure shown in FIG. 36A can prevent a large current from flowingto the pixel, so that electrostatic discharge damage of the pixel can beprevented.

FIG. 36B shows a structure for preventing electrostatic discharge damagewhen positive charges are discharged to the scan line Gi in the i-throw. A transistor 6102 functioning as a protective diode is providedbetween a scan line and a wiring 6112. Note that one protective diode isarranged here; however, a plurality of protective diodes may be arrangedin series, in parallel, or in series-parallel. The transistor 6102 is ann-channel transistor; however, a p-channel transistor may be used, andpolarity of the transistor 6102 may be the same as that of thetransistor included in the scan line driver circuit or the pixel. Afirst electrode of the transistor 6102 is connected to the scan line Giin the i-th row, a second electrode of the transistor 6102 is connectedto the wiring 6112, and a gate electrode of the transistor 6102 isconnected to the wiring 6112. Note that a potential higher than H levelof the signal input to the scan line Gi in the i-th row is input to thewiring 6112. Therefore, when charges are not discharged to the scan lineGi in the i-th row, the transistor 6102 is off. On the other hand, whenpositive charges are discharged to the scan line Gi in the i-th row, thepotential of the scan line Gi in the i-th row increases instantaneously.At this time, the potential of the scan line Gi in the i-th row ishigher than the sum of a potential of the wiring 6112 and a thresholdvoltage of the transistor 6102, so that the transistor 6102 is turnedon. Thus, a current flows to the wiring 6112 through the transistor6102. Therefore, the structure shown in FIG. 36B can prevent a largecurrent from flowing to the pixel, so that electrostatic dischargedamage of the pixel can be prevented.

As shown in FIG. 36C, with a structure which combines FIGS. 36A and 36B,electrostatic discharge damage of the pixel can be prevented whenpositive or negative charges are discharged to the scan line Gi in thei-th row. Note that portions similar to FIGS. 36A and 36B are denoted bythe same reference numerals, and detailed description of the sameportions and portions having similar functions is omitted.

FIG. 37A shows a structure where a transistor 6201 functioning as aprotective diode is connected between a scan line and a storagecapacitor line. Note that one protective diode is arranged here;however, a plurality of protective diodes may be arranged in series, inparallel, or in series-parallel. The transistor 6201 is an n-channeltransistor; however, a p-channel transistor may be used. Polarity of thetransistor 6201 may be the same as that of the transistor included inthe scan line driver circuit or the pixel. Note that a wiring 6211functions as a storage capacitor line. A first electrode of thetransistor 6201 is connected to the scan line Gi in the i-th row, asecond electrode of the transistor 6201 is connected to the wiring 6211,and a gate electrode of the transistor 6201 is connected to the scanline Gi in the i-th row. Note that a potential lower than L level of thesignal input to the scan line Gi in the i-th row is input to the wiring6211. Therefore, when charges are not discharged to the scan line Gi inthe i-th row, the transistor 6210 is off. On the other hand, whennegative charges are discharged to the scan line Gi in the i-th row, thepotential of the scan line Gi in the i-th row decreases instantaneously.At this time, the potential of the scan line Gi in the i-th row is lowerthan a value obtained by subtracting a threshold voltage of thetransistor 6201 from a potential of the wiring 6211, so that thetransistor 6201 is turned on. Thus, a current flows to the wiring 6211through the transistor 6201. Therefore, the structure shown in FIG. 37Acan prevent a large current from flowing to the pixel, so thatelectrostatic discharge damage of the pixel can be prevented. Further,since the storage capacitor line is utilized as a wiring for dischargingcharges in the structure shown in FIG. 37A, a wiring is not required tobe added.

FIG. 37B shows a structure for preventing electrostatic discharge damagewhen positive charges are discharged to the scan line Gi in the i-throw. Here, a potential higher than H level of the signal input to thescan line Gi in the i-th row is input to the wiring 6211. Therefore,when charges are not discharged to the scan line Gi in the i-th row, atransistor 6202 is off. On the other hand, when positive charges aredischarged to the scan line Gi in the i-th row, the potential of thescan line Gi in the i-th row increases instantaneously. At this time,the potential of the scan line Gi in the i-th row is higher than the sumof a potential of the wiring 6211 and a threshold voltage of thetransistor 6202, so that the transistor 6202 is turned on. Thus, acurrent flows to the wiring 6211 through the transistor 6202. Therefore,the structure shown in FIG. 37B can prevent a large current from flowingto the pixel, so that electrostatic discharge damage of the pixel can beprevented. Further, since the storage capacitor line is utilized fordischarging charges in the structure shown in FIG. 37B, a wiring is notneeded to be added. Note that portions similar to FIG. 37A are denotedby the same reference numerals, and detailed description of the sameportions and portions having similar functions is omitted.

Next, FIG. 38A shows a structure for preventing electrostatic dischargedamage caused in a signal line by a protective diode. FIG. 38A shows astructure where the protective diode is provided between a wiring 6411and the signal line. Although not shown, a plurality of pixels areconnected to the signal line Sj in the j-th column. A transistor 6401 isused as the protective diode. The transistor 6401 is an n-channeltransistor; however, a p-channel transistor may be used. Polarity of thetransistor 6401 may be the same as that of a transistor included in asignal line driver circuit or the pixel.

Note that one protective diode is arranged here; however, a plurality ofprotective diodes may be arranged in series, in parallel, or inseries-parallel.

A first electrode of the transistor 6401 is connected to the signal lineSj in the j-th column, a second electrode of the transistor 6401 isconnected to the wiring 6411, and a gate electrode of the transistor6401 is connected to the signal line Sj in the j-th column.

An operation of FIG. 38A is described. A certain potential is input tothe wiring 6411, which is lower than the smallest value of a videosignal input to the signal line Sj in the j-th column. When positive ornegative charges are not discharged to the signal line Sj in the j-thcolumn, a potential of the signal line Sj in the j-th column is the sameas the video signal, so that the transistor 6401 is off. On the otherhand, when negative charges are discharged to the signal line Sj in thej-th column, the potential of the signal line Sj in the j-th columndecreases instantaneously. At this time, the potential of the signalline Sj in the j-th column is lower than a value obtained by subtractinga threshold voltage of the transistor 6401 from a potential of thewiring 6411, so that the transistor 6401 is turned on. Thus, a currentflows to the wiring 6411 through the transistor 6401. Therefore, thestructure shown in FIG. 38A can prevent a large current from flowing tothe pixel, so that electrostatic discharge damage of the pixel can beprevented.

FIG. 38B shows a structure for preventing electrostatic discharge damagewhen positive charges are discharged to the signal line Sj in the j-thcolumn. A transistor 6402 functioning as a protective diode is providedbetween the signal line and a wiring 6412. Note that one protectivediode is arranged here; however, a plurality of protective diodes may bearranged in series, in parallel, or in series-parallel. The transistor6402 is an n-channel transistor; however, a p-channel transistor may beused. Polarity of the transistor 6402 may be the same as that of thetransistor included in the signal line driver circuit or the pixel. Afirst electrode of the transistor 6402 is connected to the signal lineSj in the j-th column, a second electrode of the transistor 6402 isconnected to the wiring 6412, and a gate electrode of the transistor6402 is connected to the wiring 6412. Note that a potential higher thanthe largest value of a video signal input to the signal line Sj in thej-th column is input to the wiring 6412. Therefore, when charges are notdischarged to the signal line Sj in the j-th column, the transistor 6402is off. On the other hand, when positive charges are discharged to thesignal line Sj in the j-th column, the potential of the signal Line Sjin the j-th column increases instantaneously. At this time, thepotential of the signal line Sj in the j-th column is higher than thesum of a potential of the wiring 6412 and a threshold voltage of thetransistor 6402, so that the transistor 6402 is turned on. Thus, acurrent flows to the wiring 6412 through the transistor 6402. Therefore,the structure shown in FIG. 38B can prevent a large current from flowingto the pixel, so that electrostatic discharge damage of the pixel can beprevented.

As shown in FIG. 38C, with a structure which combines FIGS. 38A and 38B,electrostatic discharge damage of the pixel can be prevented whenpositive or negative charges are discharged to the signal line Sj in thej-th column. Note that portions similar to FIGS. 38A and 38B are denotedby the same reference numerals, and detailed description of the sameportions and portions having similar functions is omitted.

In this embodiment mode, the structures for preventing electrostaticdischarge damage of the pixel connected to the scan line and the signalline are described. However, the structure in this embodiment mode isnot only used for preventing electrostatic discharge damage of the pixelconnected to the scan line and the signal line. For example, when thisembodiment mode is used for the wiring to which a signal or a potentialis input, which is connected to the scan line driver circuit and thesignal line driver circuit shown in Embodiment Modes 1 to 4,electrostatic discharge damage of the scan line driver circuit and thesignal line driver circuit can be prevented.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be applied to or combined with the contents (or part of thecontents) described in another drawing. Further, much more drawings canbe formed by combining each part with another part in theabove-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to or combined with the contents(or part of the contents) described in a drawing in another embodimentmode. Further, much more drawings can be formed by combining each partin each drawing in this embodiment mode with part of another embodimentmode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents described in other embodiment modes, an example of related partthereof, or the like. Therefore, the contents described in otherembodiment modes can be applied to or combined with this embodimentmode.

Embodiment Mode 7

In this embodiment mode, another structure of a display device which canbe applied to each display device shown in Embodiment Modes 1 to 4 isdescribed.

FIG. 39A shows a structure where a diode-connected transistor isprovided between a scan line and another scan line. FIG. 39A shows astructure where a diode-connected transistor 6301 a is provided betweenthe scan line Gi−1 in the to (i−1)th row and the scan line Gi in thei-th row, and a diode-connected transistor 6301 b is provided betweenthe scan line Gi in the i-th row and the scan line Gi+1 in the (i+1)throw. Note that the transistors 6301 a and 6301 b are n-channeltransistors; however, p-channel transistors may be used. Polarity of thetransistors 6301 a and 6301 b may be the same as that of a transistorincluded in a scan line driver circuit or a pixel.

Note that in FIG. 39A, the scan line Gi−1 in the (i−1)th row, the scanline Gi in the i-th row, and the scan line Gi+1 in the (i+1)th row aretypically shown, and a diode-connected transistor is similarly providedbetween other scan lines.

A first electrode of the transistor 6301 a is connected to the scan lineGi in the i-th row, a second electrode of the transistor 6301 a isconnected to the scan line Gi−1 in the (i−1)th row, and a gate electrodeof the transistor 6301 a is connected to the scan line Gi−1 in the(i−1)th row. A first electrode of the transistor 6301 b is connected tothe scan line Gi+1 in the (i+1)th row, a second electrode of thetransistor 6301 b is connected to the scan line Gi in the i-th row, anda gate electrode of the transistor 6301 b is connected to the scan lineGi in the i-th row.

An operation of FIG. 39A is described. In each scan line driver circuitshown in Embodiment Modes 1 to 4, the scan line Gi−1 in the (i−1)th row,the scan line Gi in the i-th row, and the scan line Gi+1 in the (i+1)throw are kept at L level in the non-selection period. Therefore, thetransistors 6301 a and 6301 b are turned off. However, when thepotential of the scan line Gi in the i-th row is increased due to noiseor the like, for example, a pixel is selected by the scan line Gi in thei-th row and a wrong video signal is written to the pixel. Accordingly,by providing the diode-connected transistor between the scan lines asshown in FIG. 39A, writing of a wrong video signal to the pixel can beprevented. This is because when the potential of the scan line Gi in thei-th row is increased to more than the sum of a potential of the scanline Gi−1 in the (i−1)th row and a threshold voltage of the transistor6301 a, the transistor 6301 a is turned on and the potential of the scanline Gi in the i-th row is decreased; thus, a pixel is not selected bythe scan line Gi in the i-th row.

The structure of FIG. 39A is particularly advantageous when a scan linedriver circuit and a pixel portion are formed over the same substrate,because in the scan line driver circuit including only n-channeltransistors or only p-channel transistors, a scan line is sometimes in afloating state and noise is easily caused in the scan line.

FIG. 39B shows a structure where a direction of a diode-connectedtransistor provided between the scan lines is reversed to that in FIG.39A. Note that transistors 6302 a and 6302 b are n-channel transistors;however, p-channel transistors may be used. Polarity of the transistors6302 a and 6302 b may be the same as that of the transistor included inthe scan line driver circuit or the pixel. In FIG. 39B, a firstelectrode of the transistor 6302 a is connected to the scan line Gi inthe i-th row, a second electrode of the transistor 6302 a is connectedto the scan line Gi−1 in the (i−1)th row, and a gate electrode of thetransistor 6302 a is connected to the scan line Gi in the i-th row. Afirst electrode of the transistor 6302 b is connected to the scan lineGi+1 in the (i+1)th row, a second electrode of the transistor 6302 b isconnected to the scan line Gi in the i-th row, and a gate electrode ofthe transistor 6302 b is connected to the scan line Gi+1 in the (i+1)throw. In FIG. 39B, similarly to FIG. 38A, when the potential of the scanline Gi in the i-th row is increased to more than the sum of thepotential of the scan line Gi+1 in the (i+1)th row and a thresholdvoltage of the transistor 6302 b, the transistor 6302 b is turned on andthe potential of the scan line Gi in the i-th row is decreased. Thus, apixel is not selected by the scan line Gi in the i-th row, and writingof a wrong video signal to the pixel can be prevented.

As shown in FIG. 39C, with a structure which combines FIGS. 39A and 39B,even when the potential of the scan line Gi in the i-th row isincreased, the transistors 6301 a and 6301 b are tuned on, so that thepotential of the scan line Gi in the i-th row is decreased. Note that inFIG. 39C, since a current flows through two transistors, larger noisecan be removed. Note that portions similar to FIGS. 1539A and 39B aredenoted by the same reference numerals, and detailed description of thesame portions and portions having similar functions is omitted.

Note that as shown in FIGS. 37A and 37B, when a diode-connectedtransistor is provided between the scan line and the storage capacitorline, advantageous effects similar to FIGS. 39A, 39B, and 39C can beobtained.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be applied to or combined with the contents (or part of thecontents) described in another drawing. Further, much more drawings canbe formed by combining each part with another part in theabove-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to or combined with the contents(or part of the contents) described in a drawing in another embodimentmode. Further, much more drawings can be formed by combining each partin each drawing in this embodiment mode with part of another embodimentmode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents described in other embodiment modes, an example of related partthereof, or the like. Therefore, the contents described in otherembodiment modes can be applied to or combined with this embodimentmode.

Embodiment Mode 8

In this embodiment mode, a structure and a manufacturing method of atransistor are described.

FIG. 40A shows a structure example of a transistor. FIGS. 40B to 40Gshow an example of a manufacturing method of the transistor.

Note that the structure and the manufacturing method of a transistor arenot limited to those shown in FIGS. 40A to 40G, and various structuresand manufacturing methods can be employed.

First, a structure example of a transistor is described with referenceto FIG. 40A. FIG. 40A is a cross-sectional view of a plurality oftransistors each having a different structure. Here, in FIG. 40A, theplurality of transistors each having a different structure arejuxtaposed, which is for describing structures of the transistors.Therefore, the transistors are not needed to be actually juxtaposed asshown in FIG. 40A and can be separately formed as needed.

Next, characteristics of each layer forming the transistor aredescribed.

A substrate 110111 can be a glass substrate using barium borosilicateglass, alumino borosilicate glass, or the like, a quartz substrate, aceramic substrate, a metal substrate containing stainless steel, or thelike. In addition, a substrate formed of plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN), orpolyethersulfone (PES), or a substrate formed of a flexible syntheticresin such as acrylic can also be used. By using a flexible substrate, asemiconductor device capable of being bent can be formed. A flexiblesubstrate has no strict limitations on an area or a shape of thesubstrate. Therefore, for example, when a substrate having a rectangularshape, each side of which is 1 meter or more, is used as the substrate110111, productivity can be significantly improved. Such an advantage ishighly favorable as compared with the case where a circular siliconsubstrate is used.

An insulating film 110112 functions as a base film and is provided toprevent alkali metal such as Na or alkaline earth metal from thesubstrate 110111 from adversely affecting characteristics of asemiconductor element. The insulating film 110112 can have asingle-layer structure or a stacked-layer structure of an insulatingfilm containing oxygen or nitrogen, such as silicon oxide (SiOx),silicon nitride (SiNx), silicon oxynitride (SiOxNy) (x>y), or siliconnitride oxide (SiNxOy) (x>y). For example, when the insulating film110112 is provided to have a two-layer structure, it is preferable thata silicon nitride oxide film be used as a first insulating film and asilicon oxynitride film be used as a second insulating film. As anotherexample, when the insulating film 110112 is provided to have athree-layer structure, it is preferable that a silicon oxynitride filmbe used as a first insulating film, a silicon nitride oxide film be usedas a second insulating film, and a silicon oxynitride film be used as athird insulating film.

Semiconductor layers 110113, 110114, and 110115 can be formed using anamorphous semiconductor, a microcrystalline semiconductor, or asemi-amorphous semiconductor (SAS). Alternatively, a polycrystallinesemiconductor layer may be used. SAS is a semiconductor having anintermediate structure between amorphous and crystalline (includingsingle crystal and polycrystalline) structures and having a third statewhich is stable in free energy. Moreover, SAS includes a crystallineregion with a short-range order and lattice distortion. A crystallineregion of 0.5 to 20 nm can be observed at least in part of a film. Whensilicon is contained as a main component, Raman spectrum shifts to awave number side lower than 520 cm⁻¹. The diffraction peaks of (111) and(220) which are thought to be derived from a silicon crystalline latticeare observed by X-ray diffraction. SAS contains hydrogen or halogen ofat least 1 atomic % or more to compensate dangling bonds. SAS is formedby glow discharge decomposition (plasma CVD) of a material gas. As thematerial gas, SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like canbe used. Further, GeF₄ may be mixed. Alternatively, the material gas maybe diluted with H₂, or H₂ and one or more kinds of rare gas elementsselected from He, Ar, Kr, and Ne. A dilution ratio is in the range of 2to 1000 times. Pressure is in the range of approximately 0.1 to 133 Pa,and a power supply frequency is 1 to 120 MHz, preferably 13 to 60 MHz. Asubstrate heating temperature may be 300° C. or lower. A concentrationof impurities in atmospheric components such as oxygen, nitrogen, andcarbon is preferably 1×10²⁰ cm⁻¹ or less as impurity elements in thefilm. In particular, an oxygen concentration is 5×10¹⁹/cm³ or less,preferably 1×10¹⁹/cm³ or less. Here, an amorphous silicon layer isformed using a material containing silicon (Si) as its main component(e.g., Si_(x)Ge_(1-x): 0<x<1) by a sputtering method, an LPCVD method, aplasma CVD method, or the like. Then, the amorphous silicon layer iscrystallized by a crystallization method such as a laser crystallizationmethod, a thermal crystallization method using RTA or an annealingfurnace, or a thermal crystallization method using a metal element whichpromotes crystallization.

An insulating film 110116 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx), siliconoxynitride (SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y).

A gate electrode 110117 can have a single-layer structure of aconductive film or a stacked-layer structure of two or three conductivefilms. As a material for the gate electrode 110117, for example, asingle film of an element such as tantalum (Ta), titanium (Ti),molybdenum (Mo), tungsten (W), chromium (Cr), silicon (Si), or the like;a nitride film containing the aforementioned element (typically, atantalum nitride film, a tungsten nitride film, or a titanium nitridefilm); an alloy film in which the aforementioned elements are combined(typically, a Mo—W alloy or a Mo—Ta alloy); a silicide film containingthe aforementioned element (typically, a tungsten silicide film or atitanium silicide film); and the like can be used. Note that theaforementioned single film, nitride film, alloy film, silicide film, andthe like can have a single-layer structure or a stacked-layer structure.

An insulating film 110118 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx), siliconoxynitride (SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y); or afilm containing carbon, such as a DLC (Diamond-Like Carbon), by asputtering method, a plasma CVD method, or the like.

An insulating film 110119 can have a single-layer structure or astacked-layer structure of a siloxane resin; an insulating filmcontaining oxygen or nitrogen, such as silicon oxide (SiOx), siliconnitride (SiNx), silicon oxynitride (SiOxNy) (x>y), or silicon nitrideoxide (SiNxOy) (x>y); a film containing carbon, such as a DLC(Diamond-Like Carbon); or an organic material such as epoxy, polyimide,polyamide, polyvinyl phenol, benzocyclobutene, or acrylic. Note that asiloxane resin corresponds to a resin having Si—O—Si bonds. Siloxaneincludes a skeleton structure of a bond of silicon (Si) and oxygen (O).As a substituent, an organic group containing at least hydrogen (such asan alkyl group or an aryl group) is used. Alternatively, a fluoro group,or a fluoro group and an organic group containing at least hydrogen canbe used as a substituent. Note that the insulating film 110119 can beprovided to cover the gate electrode 110117 directly without provisionof the insulating film 110118.

As a conductive film 110123, a single film of an element such as Al, Ni,C, W, Mo, Ti, Pt, Cu, Ta, Au, Mn, or the like, a nitride film containingthe aforementioned element, an alloy film in which the aforementionedelements are combined, a silicide film containing the aforementionedelement, or the like can be used. For example, as an alloy containingthe plurality of elements, an Al alloy containing C and Ti, an Al alloycontaining Ni, an Al alloy containing C and Ni, an Al alloy containing Cand Mn, or the like can be used. When the conductive film has astacked-layer structure, a structure can be such that Al is interposedbetween Mo, Ti, or the like; thus, resistance of Al to heat and chemicalreaction can be improved.

Next, characteristics of each structure are described with reference tothe cross-sectional view of the plurality of transistors each having adifferent structure in FIG. 40A.

A transistor 110101 is a single drain transistor. Since it can be formedby a simple method, it is advantageous in low manufacturing cost andhigh yield. Here, the semiconductor layers 110113 and 110115 havedifferent concentrations of impurities, and the semiconductor layer110113 is used as a channel region and the semiconductor layers 110115are used as a source region and a drain region. By controlling theconcentration of impurities in this manner, resistivity of thesemiconductor layer can be controlled. Further, an electrical connectionstate of the semiconductor layer and the conductive film 110123 can becloser to ohmic contact. Note that as a method of separately forming thesemiconductor layers each having different concentration of impurities,a method where impurities are doped in the semiconductor layer using thegate electrode 110117 as a mask can be used.

In a transistor 110102, the gale electrode 110117 has a tapered angle.Here, the tapered angle is 45° or more and less than 95°, andpreferably, 60° or more and less than 95°. Note that the tapered angelmay be less than 45°. Here, the semiconductor layers 110113, 110114, and110115 have different concentrations of impurities. The semiconductorlayer 110113 is used as a channel region, the semiconductor layers110114 as lightly doped drain (LDD) regions, and the semiconductorlayers 110115 as a source region and a drain region. By controlling theconcentration of impurities in this manner, resistivity of thesemiconductor layer can be controlled. Further, an electrical connectionstate of the semiconductor layer and the conductive film 110123 can becloser to ohmic contact. Moreover, since the transistor includes the LDDregions, high electric field is hardly applied inside the transistor, sothat deterioration of the element due to hot carriers can be suppressed.Note that as a method of separately forming the semiconductor layershaving different concentrations of impurities, a method where impuritiesare doped in the semiconductor layer using the gate electrode 110117 asa mask can be used. In the transistor 110102, since the gate electrode110117 has a tapered angle, gradient of the concentration of impuritiesdoped in the semiconductor layer through the gate electrode 110117 canbe provided, and the LDD region can be easily formed. Thus, it isadvantageous in low manufacturing cost and high yield.

A transistor 110103 has a structure where the gate electrode 110117 isformed of at least two layers and a lower gate electrode is longer thanan upper gate electrode. In this specification, such a shape of thelower and upper gate electrodes is called a hat shape. When the gateelectrode 110117 has a hat shape, an LDD region can be formed withoutaddition of a photomask. Note that a structure where the LDD regionoverlaps with the gate electrode 110117, like the transistor 110103, isparticularly called a GOLD (Gate Overlapped LDD) structure. As a methodof forming the gate electrode 110117 with a hat shape, the followingmethod may be used.

First, when the gate electrode 110117 is patterned, the lower and uppergate electrodes are etched by dry etching so that side surfaces thereofare inclined (tapered). Then, an inclination of the upper gate electrodeis processed to be almost perpendicular by anisotropic etching. Thus,the gate electrode a cross section of which is a hat shape is formed.After that, impurity elements are doped twice, so that the semiconductorlayer 110113 used as the channel region, the semiconductor layers 110114used as the LDD regions, and the semiconductor layers 110115 used as asource electrode and a drain electrode are formed.

Note that part of the LDD region, which overlaps with the gate electrode110117, is referred to as an Lov region, and part of the LDD region,which does not overlap with the gate electrode 110117, is referred to asan Loff region. The Loff region is highly effective in suppressing anoff-current value, whereas it is not very effective in preventingdeterioration in an on-current value due to hot carriers by relieving anelectric field in the vicinity of the drain. On the other hand, the Lovregion is highly effective in preventing deterioration in the on-currentvalue by relieving the electric field in the vicinity of the drain,whereas it is not very effective in suppressing the off-current value.Thus, it is preferable to form a transistor having a structureappropriate for characteristics of each of the various circuits. Forexample, when a semiconductor device is used for a display device, atransistor having an Loff region is preferably used as a pixeltransistor in order to suppress the off-current value. On the otherhand, as a transistor in a peripheral circuit, a transistor having anLov region is preferably used in order to prevent deterioration in theon-current value by relieving the electric field in the vicinity of thedrain.

A transistor 110104 includes a sidewall 110121 in contact with the sidesurface of the gate electrode 110117. When the transistor includes thesidewall 110121, a region overlapping with the sidewall 110121 can bemade to be an LDD region.

In a transistor 110105, an LDD (Lai) region is formed by doping in thesemiconductor layer with use of a mask. Thus, the LDD region can surelybe formed, and an off-current value of the transistor can be reduced.

In a transistor 110106, an LDD (Lov) region is formed by doping in thesemiconductor layer with use of a mask. Thus, the LDD region can surelybe formed, and deterioration in an on-current value can be prevented byrelieving the electric field in the vicinity of the drain of thetransistor.

Next, an example of a method for manufacturing a transistor is describedwith reference to FIGS. 40B to 40G.

In this embodiment mode, surfaces of the substrate 110111, theinsulating film 110112, the semiconductor layers 110113, 110114, and110115, the insulating film 110116, the insulating film 110118, or theinsulating film 110119 are oxidized or nitrided by plasma treatment, sothat the semiconductor layer or the insulating film can be oxidized ornitrided. By oxidizing or nitriding the semiconductor layer or theinsulating film by plasma treatment in such a manner, a surface of thesemiconductor layer or the insulating film is modified, and theinsulating film can be formed to be denser than an insulating filmformed by a CVD method or a sputtering method. Thus, a defect such as apinhole can be suppressed, and characteristics and the like of asemiconductor device can be improved.

Silicon oxide (SiOx) or silicon nitride (SiNx) can be used for thesidewall 110121. As a method of forming the sidewall 110121 on the sidesurface of the gate electrode 110117, a method where a silicon oxide(SiOx) film or a silicon nitride (SiNx) film is formed after the gateelectrode 110117 is formed, and then, the silicon oxide (SiOx) film orthe silicon nitride (SiNx) film is etched by anisotropic etching can beused, for example. Thus, the silicon oxide (SiOx) film or the siliconnitride (SiNx) film remains only on the side surface of the gateelectrode 110117, so that the sidewall 110121 can be formed on the sidesurface of the gate electrode 110117.

FIG. 44 shows cross-sectional structures of a bottom-gate transistor anda capacitor.

A first insulating film (an insulating film 110502) is formed over anentire substrate 110501. Note that the structure is not limited thereto,and the first insulating film (the insulating film 110502) is not formedin some cases. The first insulating film can prevent impurities from thesubstrate from adversely affecting a semiconductor layer and changingproperties of a transistor. That is, the first insulating film functionsas a base film. Thus, a transistor with high reliability can be formed.As the first insulating film, a single layer or a stacked layer of asilicon oxide film, a silicon nitride film, a silicon oxynitride film(SiOxNy), or the like can be used.

A first conductive layer (a conductive layer 110503 and a conductivelayer 110504) is formed over the first insulating film. The conductivelayer 110503 includes a portion functioning as a gate electrode of atransistor 110520. The conductive layer 110504 includes a portionfunctioning as a first electrode of a capacitor 110521. As the firstconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, Ge, or the like, or an alloy of these elements can be used.Further, a stacked layer of these elements (including the alloy thereof)can be used.

A second insulating film (an insulating film 110514) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

As a portion of the second insulating film, which is in contact with thesemiconductor layer, a silicon oxide film is preferably used. This isbecause the trap level at the interface between the semiconductor layerand the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A semiconductor layer is formed in part of a portion over the secondinsulating film, which overlaps with the first conductive layer, by aphotolithography method, an inkjet method, a printing method, or thelike. Part of the semiconductor layer extends to a portion over thesecond insulating film, which does not overlap with the first conductivelayer. The semiconductor layer includes a channel formation region (achannel formation region 110510), an LDD region (LDD regions 110508 and110509), and an impurity region (impurity regions 110505, 110506, and110507). The channel formation region 110510 functions as a channelformation region of the transistor 110520. The LDD regions 110508 and110509 function as LDD regions of the transistor 110520. Note that theLDD regions 110508 and 110509 are not necessarily formed. The impurityregion 110505 includes a portion functioning as one of a sourceelectrode and a drain electrode of the transistor 110520. The impurityregion 110506 includes a portion functioning as the other of the sourceelectrode and the drain electrode of the transistor 110520. The impurityregion 110507 includes a portion functioning as a second electrode ofthe capacitor 110521.

A third insulating film (an insulating film 110511) is formed entirelyover the impurity region 110505, the LDD region 110508, the channelformation region 110510, the LDD region 110509, the impurity region110506, the second insulating film 110514, and the impurity region110507. A contact hole is selectively formed in part of the thirdinsulating film. The insulating film 110511 functions as an interlayerfilm. As the third insulating film, an inorganic material (e.g., siliconoxide, silicon nitride, or silicon oxynitride), an organic compoundmaterial having a low dielectric constant (e.g., a photosensitive ornonphotosensitive organic resin material), or the like can be used.Alternatively, a material including siloxane may be used. Note thatsiloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substitute, an organic groupcontaining at least hydrogen (such as an alkyl group or an aryl group)is used. Alternatively, a fluoro group, or a fluoro group and an organicgroup containing at least hydrogen may be used as a substituent.

A second conductive layer (a conductive layer 110512 and a conductivelayer 110513) is formed over the third insulating film. The conductivelayer 110512 is connected to the other of the source electrode and thedrain electrode of the transistor 110520 through the contact hole formedin the third insulating film. Thus, the conductive layer 110512 includesa portion functioning as the other of the source electrode and the drainelectrode of the transistor 110520. When the conductive layer 110513 iselectrically connected to the conductive layer 110504, the conductivelayer 110513 includes a portion functioning as the first electrode ofthe capacitor 110521. Alternatively, when the conductive layer 110513 iselectrically connected to the conductive layer 110507, the conductivelayer 110513 includes a portion functioning as the second electrode ofthe capacitor 110521. Further alternatively, when the conductive layer110513 is not connected to the conductive layers 110504 and 110507,another capacitor is formed other than the capacitor 110521. In thiscapacitor, the conductive layer 110513, the conductive layer 110507, andthe insulating film 110511 are used as a first electrode, a secondelectrode, and an insulating film, respectively. Note that as the secondconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, Ge, or the like, or an alloy of these elements can be used.Further, a stacked layer of these elements (including the alloy thereof)can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

Next, structures of a transistor and a capacitor are described in thecase where an amorphous silicon (a-Si) film, a microcrystal siliconfilm, or the like is used as a semiconductor layer of the transistor.

FIG. 41 shows cross-sectional structures of a top-gate transistor and acapacitor.

A first insulating film (an insulating film 110202) is formed over anentire substrate 110201. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing properties of a transistor. That is, the firstinsulating film functions as a base film. Thus, a transistor with highreliability can be formed. As the first insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand manufacturing cost can be realized. Further, since the structure canbe simplified, the yield can be improved.

A first conductive layer (a conductive layer 110203, a conductive layer110204, and a conductive layer 110205) is formed over the firstinsulating film. The conductive layer 110203 includes a portionfunctioning as one of a source electrode and a drain electrode of atransistor 110220. The conductive layer 110204 includes a portionfunctioning as the other of the source electrode and the drain electrodeof the transistor 110220. The conductive layer 110205 includes a portionfunctioning as a first electrode of a capacitor 110221. As the firstconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, Ge, or the like, or an alloy of these elements can be used.Further, a stacked layer of these elements (including the alloy thereof)can be used.

A first semiconductor layer (a semiconductor layer 110206 and asemiconductor layer 110207) is formed above the conductive layers 110203and 110204. The semiconductor layer 110206 includes a portionfunctioning as one of the source electrode and the drain electrode. Thesemiconductor layer 110207 includes a portion functioning as the otherof the source electrode and the drain electrode. As the firstsemiconductor layer, silicon containing phosphorus or the like can beused.

A second semiconductor layer (a semiconductor layer 110208) is formedover the first insulating film and between the conductive layer 110203and the conductive layer 110204. Part of the semiconductor layer 110208extends over the conductive layers 110203 and 110204. The semiconductorlayer 110208 includes a portion functioning as a channel region of thetransistor 110220. As the second semiconductor layer, a semiconductorlayer having no crystallinity such as amorphous silicon (a-Si:H), asemiconductor layer such as microcrystal (μ-Si:H), or the like can beused.

A second insulating film (an insulating film 110209 and an insulatingfilm 110210) is formed to cover at least the semiconductor layer 110208and the conductive layer 110205. The second insulating film functions asa gate insulating film. As the second insulating film, a single layer ora stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

As a portion of the second insulating film, which is in contact with thesecond semiconductor layer, a silicon oxide film is preferably used.This is because the trap level at the interface between the secondsemiconductor layer and the second insulating film is lowered.

Note that when the second insulating film is in contact with Mo, asilicon oxide film is preferably used as a portion of the secondinsulating film in contact with Mo. This is because the silicon oxidefilm does not oxidize Mo.

A second conductive layer (a conductive layer 110211 and a conductivelayer 110212) is formed over the second insulating film. The conductivelayer 110211 includes a portion functioning as a gate electrode of thetransistor 110220. The conductive layer 110212 functions as a secondelectrode of the capacitor 110221 or a wiring. As the second conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or the like, or an alloy of these elements can be used. Further, astacked layer of these elements (including the alloy thereof) can beused.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed. FIG. 42 showscross-sectional structures of an inversely staggered (bottom gate)transistor and a capacitor. In particular, the transistor shown in FIG.42 has a channel etch structure.

A first insulating film (an insulating film 110302) is formed over anentire substrate 110301. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing properties of a transistor. That is, the firstinsulating film functions as a base film. Thus, a transistor with highreliability can be formed. As the first insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand manufacturing cost can be realized. Further, since the structure canbe simplified, the yield can be improved.

A first conductive layer (a conductive layer 110303 and a conductivelayer 110304) is formed over the first insulating film. The conductivelayer 110303 includes a portion functioning as a gate electrode of atransistor 110320. The conductive layer 110304 includes a portionfunctioning as a first electrode of a capacitor 110321. As the firstconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, Ge, or the like, or an alloy of these elements can be used.Further, a stacked layer of these elements (including the alloy thereof)can be used.

A second insulating film (an insulating film 110305) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

As a portion of the second insulating film, which is in contact with thesemiconductor layer, a silicon oxide film is preferably used. This isbecause the trap level at the interface between the semiconductor layerand the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 110306) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer110306 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer110306 includes a portion functioning as a channel region of thetransistor 110320. As the semiconductor layer 110306, a semiconductorlayer having no crystallinity such as amorphous silicon (a-Si:H), asemiconductor layer such as microcrystal (μ-Si:H), or the like can beused.

A second semiconductor layer (a semiconductor layer 110307 and asemiconductor layer 110308) is formed over part of the firstsemiconductor layer. The semiconductor layer 110307 includes a portionfunctioning as one of a source electrode and a drain electrode. Thesemiconductor layer 110308 includes a portion functioning as the otherof the source electrode and the drain electrode. As the secondsemiconductor layer, silicon containing phosphorus or the like can beused.

A second conductive layer (a conductive layer 110309, a conductive layer110310, and a conductive layer 110311) is formed over the secondsemiconductor layer and the second insulating film. The conductive layer110309 includes a portion functioning as one of a source electrode and adrain electrode of the transistor 110320. The conductive layer 110310includes a portion functioning as the other of the source electrode andthe drain electrode of the transistor 110320. The conductive layer110311 includes a portion functioning as a second electrode of thecapacitor 110321. As the second conductive layer, Ti, Mo, Ta, Cr, W, Al,Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or the like, or an alloy ofthese elements can be used. Further, a stacked layer of these elements(including the alloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Here, an example of a process of forming a channel etch type transistoris described. The first semiconductor layer and the second semiconductorlayer can be formed using the same mask. Specifically, the firstsemiconductor layer and the second semiconductor layer are sequentiallyformed. At this time, the first semiconductor layer and the secondsemiconductor layer are formed using the same mask.

Another example of a process of forming a channel etch type transistoris described. Without using an additional mask, a channel region of atransistor can be formed. Specifically, after the second conductivelayer is formed, part of the second semiconductor layer is removed usingthe second conductive layer as a mask. Alternatively, part of the secondsemiconductor layer is removed by using the same mask as the secondconductive layer. The first semiconductor layer below the removed secondsemiconductor layer functions as a channel region of the transistor.FIG. 43 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 43 has a channel protection (channel stop) structure.

A first insulating film (an insulating film 110402) is formed over anentire substrate 110401. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing properties of a transistor. That is, the firstinsulating film functions as a base film. Thus, a transistor with highreliability can be formed. As the first insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiOxNy), or the like can be used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand manufacturing cost can be realized. Further, since the structure canbe simplified, the yield can be increased.

A first conductive layer (a conductive layer 110403 and a conductivelayer 110404) is formed over the first insulating film. The conductivelayer 110403 includes a portion functioning as a gate electrode of atransistor 110420. The conductive layer 110404 includes a portionfunctioning as a first electrode of a capacitor 110421. As the firstconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, Ge, or the like, or an alloy of these elements can be used.Further, a stacked layer of these elements (including the alloy thereof)can be used.

A second insulating film (an insulating film 110405) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

As a portion of the second insulating film, which is in contact with thesemiconductor layer, a silicon oxide film is preferably used. This isbecause the trap level at the interface between the semiconductor layerand the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 110406) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer110406 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer110406 includes a portion functioning as a channel region of thetransistor 110420. As the semiconductor layer 110406, a semiconductorlayer having no crystallinity such as amorphous silicon (a-Si:H), asemiconductor layer such as microcrystal (μ-Si:H), or the like can beused.

A third insulating film (an insulating film 110412) is formed over partof the first semiconductor layer. The insulating film 110412 has afunction to prevent the channel region of the transistor 110420 frombeing removed by etching. That is, the insulating film 110412 functionsas a channel protection film (a channel stop film). As the thirdinsulating film, a single layer or a stacked layer of a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film (SiOxNy), or thelike can be used.

A second semiconductor layer (a semiconductor layer 110407 and asemiconductor layer 110408) is formed over part of the firstsemiconductor layer and part of the third insulating film. Thesemiconductor layer 110407 includes a portion functioning as one of asource electrode and a drain electrode. The semiconductor layer 110408includes a portion functioning as the other of the source electrode andthe drain electrode. As the second semiconductor layer, siliconcontaining phosphorus or the like can be used.

A second conductive layer (a conductive layer 110409, a conductive layer110410, and a conductive layer 110411) is formed over the secondsemiconductor layer. The conductive layer 110409 includes a portionfunctioning as one of the source electrode and the drain electrode ofthe transistor 110420. The conductive layer 110410 includes a portionfunctioning as the other of the source electrode and the drain electrodeof the transistor 110420. The conductive layer 110411 includes a portionfunctioning as a second electrode of the capacitor 110421. As the secondconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, Ge, or the like, or an alloy of these elements can be used.Further, a stacked layer of these elements (including the alloy thereof)can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

The above is the description of the structures and manufacturing methodsof transistors. Here, a wiring, an electrode, a conductive layer, aconductive film, a terminal, a via, a plug, and the like are preferablyformed of one or more elements selected from aluminum (Al), tantalum(Ta), titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd),chromium (Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag),copper (Cu), magnesium (Mg), scandium (Sc), cobalt (Co), zinc (Zn),niobium (Nb), silicon (Si), phosphorus (P), boron (B), arsenic (As),gallium (Ga), indium (In), tin (Sn), and oxygen (O); or a compound or analloy material including one or more of the aforementioned elements(e.g., indium tin oxide (ITO), indium zinc oxide (IZO), indium tin oxidecontaining silicon oxide (ITSO), zinc oxide (ZnO), tin oxide (SnO),cadmium tin oxide (CTO), aluminum neodymium (Al—Nd), magnesium silver(Mg—Ag), or molybdenum-niobium (Mo—Nb)); a substance in which thesecompounds are combined; or the like. Alternatively, they are preferablyformed to contain a substance including a compound (silicide) of siliconand one or more of the aforementioned elements (e.g., aluminum silicon,molybdenum silicon, or nickel silicide); or a compound of nitrogen andone or more of the aforementioned elements (e.g., titanium nitride,tantalum nitride, or molybdenum nitride).

Silicon (Si) may include an n-type impurity (such as phosphorus) or ap-type impurity (such as boron). When silicon contains the impurity, theconductivity is increased, and a function similar to a general conductorcan be realized. Thus, such silicon can be utilized easily as a wiring,an electrode, or the like.

Silicon with various levels of crystallinity, such as single crystallinesilicon, polycrystalline silicon, or microcrystalline silicon can beused. Alternatively, silicon having no crystallinity, such as amorphoussilicon can be used. By using single crystalline silicon orpolycrystalline silicon, resistance of a wiring, an electrode, aconductive layer, a conductive film, a terminal, or the like can bereduced. By using amorphous silicon or microcrystalline silicon, awiring or the like can be formed by a simple process.

Aluminum and silver have high conductivity, and thus can reduce a signaldelay. Further, since aluminum and silver can be easily etched, they canbe easily patterned and minutely processed.

Copper has high conductivity, and thus can reduce a signal delay. Whencopper is used, a stacked-layer structure is preferably employed sincecopper increases adhesion.

Molybdenum and titanium are preferable since even if molybdenum ortitanium is in contact with an oxide semiconductor (e.g., ITO or IZO) orsilicon, molybdenum or titanium does not cause defects. Further,molybdenum and titanium are easily etched and has high heat resistance.

Tungsten is preferable since it has an advantage such as high heatresistance.

Neodymium is also preferable since it has an advantage such as high heatresistance. In particular, an alloy of neodymium and aluminum ispreferable since heat resistance is increased and aluminum hardly causeshillocks.

Silicon can be formed at the same time as a semiconductor layer includedin a transistor. Silicon is preferable since it has an advantage such ashigh heat resistance.

Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO),and cadmium tin oxide (CTO) have light-transmitting properties, they canbe used as a portion which transmits light. For example, they can beused for a pixel electrode or a common electrode.

IZO is preferable since it is easily etched and processed. In etchingIZO, a residue is hardly left. Thus, when IZO is used for a pixelelectrode, defects (such as short circuit or orientation disorder) of aliquid crystal element or a light-emitting element can be reduced.

A wiring, an electrode, a conductive layer, a conductive film, aterminal, a via, a plug, or the like may have a single-layer structureor a multi-layer structure. By employing a single-layer structure, eachmanufacturing process of a wiring, an electrode, a conductive layer, aconductive film, a terminal, or the like can be simplified, the numberof steps can be reduced, and cost can be reduced. Alternatively, byemploying a multi-layer structure, a wiring, an electrode, and the likewith high quality can be formed while an advantage of each material isutilized and a disadvantage thereof is reduced. For example, when alow-resistant material (e.g., aluminum) is included in a multi-layerstructure, reduction in resistance of a wiring can be realized. Asanother example, when a stacked-layer structure where a lowheat-resistant material is interposed between high heat-resistantmaterials is employed, heat resistance of a wiring, an electrode, andthe like can be increased, utilizing advantages of the lowheat-resistance material. For example, it is preferable to employ astacked-layer structure where a layer containing aluminum is interposedbetween layers containing molybdenum, titanium, neodymium, or the like.

When wirings, electrodes, or the like are in direct contact with eachother, they adversely affect each other in some cases. For example, onewiring or one electrode is mixed into a material of another wiring oranother electrode and changes its properties, and thus, an intendedfunction cannot be obtained in some cases. As another example, when ahigh-resistant portion is formed, a problem may occur so that it cannotbe normally formed. In such cases, a reactive material is preferablyinterposed by or covered with a non-reactive material in a stacked-layerstructure. For example, when ITO and aluminum are connected, titanium,molybdenum, or an alloy of neodymium is preferably interposed betweenITO and aluminum. As another example, when silicon and aluminum areconnected, titanium, molybdenum, or an alloy of neodymium is preferablyinterposed between silicon and aluminum.

The term “wiring” indicates provision of a conductor, A wiring may beextended linearly or may be short without extension. Therefore, anelectrode is included in a wiring.

Note that a carbon nanotube may be used for a wiring, an electrode, aconductive layer, a conductive film, a terminal, a via, a plug, or thelike. Since a carbon nanotube has light-transmitting properties, it canbe used for a portion which transmits light. For example, a carbonnanotube can be used for a pixel electrode or a common electrode.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 9

In this embodiment mode, a structure of a display device is described.

A structure of a display device is described with reference to FIG. 47A.FIG. 47A is a top plan view of the display device.

A pixel portion 170101, a scan line input terminal 170103, and a signalline input terminal 170104 are formed over a substrate 170100. Scanlines extending in a row direction from the scan line input terminal170103 are formed over the substrate 170100, and signal lines extendingin a column direction from the signal line input terminal 170104 areformed over the substrate 170100. Pixels 170102 are arranged in matrixin a region of the pixel portion 170101, in which the scan lines and thesignal lines are crossed.

The above is the description of the case where a signal is input from anexternal driver circuit; however, the invention is not limited thereto,and an IC chip can be mounted on a display device.

For example, as shown in FIG. 48A, an IC chip 170201 can be mounted onthe substrate 170100 by a COG (Chip On Glass) method. In this case, theIC chip 170201 can be examined before being mounted on the substrate170100, so that improvement in yield and reliability of the displaydevice can be realized. Note that portions common to those in FIG. 47Aare denoted by common reference numerals, and description thereof isomitted.

As another example, as shown in FIG. 48B, the IC chip 170201 can bemounted on an FPC (Flexible Printed Circuit) 170200 by a TAB (TapeAutomated Bonding) method. In this case, the IC chip 170201 can beexamined before being mounted on the FPC 170200, so that improvement inyield and reliability of the display device can be realized. Note thatportions common to those in FIG. 47A are denoted by common referencenumerals, and description thereof is omitted.

Not only the IC chip can be mounted on the substrate 170100, but also adriver circuit can be formed over the substrate 170100.

For example, as shown in FIG. 47B, a scan line driver circuit 170105 canbe formed over the substrate 170100. In this case, the cost can bereduced by reduction in the number of components. Further, reliabilitycan be improved by reduction in the number of connection points betweencomponents. Since the driving frequency of the scan line driver circuit170105 is low, the scan line driver circuit 170105 can be easily formedusing amorphous silicon or microcrystalline silicon as a semiconductorlayer of a transistor. Note that an IC chip for outputting a signal tothe signal line may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for outputting a signal to thesignal line is mounted by a TAB method may be provided on the substrate170100. In addition, an IC chip for controlling the scan line drivercircuit 170105 may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for controlling the scan linedriver circuit 170105 is mounted by a TAB method may be provided on thesubstrate 170100. Note that portions common to those in FIG. 47A aredenoted by common reference numerals, and description thereof isomitted.

As another example, as shown in FIG. 47C, the scan line driver circuit170105 and a signal line driver circuit 170106 can be formed over thesubstrate 170100. Thus, the cost can be reduced by reduction in thenumber of components. Further, reliability can be improved by reductionin the number of connection points between components. Note that an ICchip for controlling the scan line driver circuit 170105 may be mountedon the substrate 170100 by a COG method. Alternatively, an FPC on whichan IC chip for controlling the scan line driver circuit 170105 ismounted by a TAB method may be provided on the substrate 170100.Further, an IC chip for controlling the signal line driver circuit170106 may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for controlling the signalline driver circuit 170106 is mounted by a TAB method may be provided onthe substrate 170100. Note that portions common to those in FIG. 47A aredenoted by common reference numerals, and description thereof isomitted.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 10

In this embodiment mode, a method for driving a display device isdescribed. In particular, a method for driving a liquid crystal displaydevice is described.

A liquid crystal display panel which can be used for a liquid crystaldisplay device described in this embodiment mode has a structure inwhich a liquid crystal material is interposed between two substrates.Each of the two substrates is provided with an electrode for controllingan electric field applied to the liquid crystal material. A liquidcrystal material corresponds to a material, the optical and electricalproperties of which are changed by an electric field externally applied.Accordingly, a liquid crystal panel corresponds to a device in whichdesired optical and electrical properties can be obtained by controllingvoltage applied to the liquid crystal material using the electrodeincluded in each of the two substrates. In addition, a large number ofelectrodes are arranged in a planar manner, each of the electrodescorresponds to a pixel, and voltages applied to the pixels areindividually controlled; therefore, a clear image can be displayed on aliquid crystal display panel.

Here, response time of the liquid crystal material due to change in anelectric field depends on a gap (a cell gap) between the two substratesand a type or the like of the liquid crystal material, and is generallyseveral milliseconds to several ten milliseconds. When the amount ofchange in the electric field is small, the response time of the liquidcrystal material is further lengthened. This characteristic causesdefects in image display, such as an after image, a phenomenon in whichtraces can be seen, and decrease in contrast when the liquid crystalpanel displays a moving image. In particular, when a half tone ischanged into another half tone (when change in the electric field issmall), a degree of the above-described defects become noticeable.

On the other hand, as a particular problem of a liquid crystal panelusing an active matrix method, fluctuation in writing voltage due toconstant charge driving is given. Constant charge driving in thisembodiment mode is described below.

A pixel circuit using an active matrix method includes a switch whichcontrols writing and a capacitor which holds a charge. A method fordriving the pixel circuit using the active matrix method corresponds toa method in which predetermined voltage is written in a pixel circuitwith a switch in an on state, and immediately after that, a charge inthe pixel circuit is held (a hold state) with the switch in an offstate. At the time of the hold state, exchange of the charge betweeninside and outside of the pixel circuit is not performed (a constantcharge). In general, period when the switch is in an off state isapproximately several hundreds (the number of scan lines) of timeslonger than a period when the switch is in an on state. Accordingly, itmay be considered that the switch of the pixel circuit be almost alwaysin an off state. As described above, constant charge driving in thisembodiment mode corresponds to a driving method in which a pixel circuitis in a hold state in almost all periods in driving a liquid crystalpanel.

Next, electrical properties of the liquid crystal material aredescribed. A dielectric constant as well as optical properties of theliquid crystal material are changed when an electric field externallyapplied is changed. That is, when it is considered that each pixel ofthe liquid crystal panel be a capacitor (a liquid crystal element)interposed between two electrodes, the capacitor corresponds to acapacitor, capacitance of which is changed in accordance with appliedvoltage. This phenomenon is called dynamic capacitance.

When a capacitor, the capacitance of which is changed in accordance withapplied voltage in this manner is driven by the constant charge driving,the following problem occurs. When capacitance of a liquid crystalelement is changed in a hold state in which a charge is not moved,applied voltage is also changed. This can be understood from the factthat the amount of charges is constant in a relational expression of(the amount of charges)=(capacitance)×(applied voltage).

Because of the above-described reasons, voltage at the time of a holdstate is changed from voltage at the time of writing since constantcharge driving is performed in a liquid crystal panel using an activematrix method. Accordingly, change in transmittance of the liquidcrystal element is different from change in transmittance of a liquidcrystal element in a driving method which does not take a hold state.FIGS. 45A to 45C show this state. FIG. 45A shows an example ofcontrolling voltage written in a pixel circuit when time is representedby a horizontal axis and an absolute value of the voltage is representedby a vertical axis. FIG. 45B shows an example of controlling voltagewritten in the pixel circuit when time is represented by a horizontalaxis and the voltage is represented by a vertical axis. FIG. 45C showschange in transmittance of the liquid crystal element over time in thecase where the voltage shown in FIG. 45A or 45B is written in the pixelcircuit when time is represented by a horizontal axis and an absolutevalue of the voltage is represented by a vertical axis. In each of FIGS.45A to 45C, a period F indicates a period for rewriting the voltage, andtime for rewriting the voltage is denoted by t₁, t₂, t₃, and t₄.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of 0and corresponds to |V₂| in rewriting at the time of t₁, t₂, t₃, and t₄(see FIG. 45A).

Polarity of the writing voltage corresponding to image data input to theliquid crystal display device may be switched periodically (inversiondriving: see FIG. 45B). Since direct voltage can be prevented from beingapplied to a liquid crystal as much as possible by using this method,burn-in or the like caused by deterioration of the liquid crystalelement can be prevented. Note that a period of switching the polarity(an inversion period) may be the same as a period of rewriting voltage.In this case, generation of a flicker caused by inversion driving can bereduced since the inversion period is short. Further, the inversionperiod may be a period which is integral times the period of rewritingvoltage. In this case, power consumption can be reduced since theinversion period is long and frequency of writing voltage can bedecreased by changing the polarity.

FIG. 45C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 45A or 45B is applied to theliquid crystal element. Here, the voltage |V₁| is applied to the liquidcrystal element, and transmittance of the liquid crystal element afterenough time passes corresponds to TR₁. Similarly, the voltage |V₂| isapplied to the liquid crystal element, and transmittance of the liquidcrystal element after enough time passes corresponds to TR₂. When thevoltage applied to the liquid crystal element is changed from to |V₂| atthe time of t₁, transmittance of the liquid crystal element does notimmediately become TR₂ but slowly changes as shown by a dashed line30401. For example, when the period of rewriting voltage is the same asa frame period (16.7 milliseconds) of an image signal of 60 Hz, time forseveral frames is necessary until transmittance is changed to TR₂.

Note that smooth change in transmittance over time as shown in thedashed line 30401 corresponds to change in transmittance over time whenthe voltage |V₂| is accurately applied to the liquid crystal element. Inan actual liquid crystal panel, for example, a liquid crystal panelusing an active matrix method, transmittance of the liquid crystalelement does not changed over time as shown by the dashed line 30401 butgradually changes over time as shown by a solid line 30402. This isbecause voltage at the time of a hold state is changed from voltage atthe time of writing due to constant charge driving, and it is impossibleto reach intended voltage only by one writing. Accordingly, the responsetime of transmittance of the liquid crystal element becomes furtherlonger than original response time (the dashed line 30401) inappearance, so that defects in image display, such as an after image, aphenomenon in which traces can be seen, or decrease in contrast notablyoccur.

By using overdriving, it is possible to solve a phenomenon in which theresponse time in appearance becomes further longer because of shortageof writing by dynamic capacitance and constant charge driving as well aslength of the original response time of the liquid crystal element.FIGS. 46A to 46C show this state. FIG. 46A shows an example ofcontrolling voltage written in a pixel circuit when time is representedby a horizontal axis and an absolute value of the voltage is representedby a vertical axis. FIG. 46B shows an example of controlling voltagewritten in the pixel circuit when time is represented by a horizontalaxis and the voltage is represented by a vertical axis. FIG. 46C showschange in transmittance of the liquid crystal element over time in thecase where the voltage shown in FIG. 46A or 46B is written in the pixelcircuit when time is represented by a horizontal axis and an absolutevalue of the voltage is represented by a vertical axis. In each of FIGS.46A to 46C, a period F indicates a period for rewriting the voltage, andtime for rewriting the voltage is denoted by t₁, t₂, t₃, and t₄.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of0, corresponds to |V₃| in rewriting at the time of t₁, and correspondsto |V₂| in rewriting at the time of t₂, t₃, and t₄ (see FIG. 46A).

Polarity of the writing voltage corresponding to image data input to theliquid crystal display device may be switched periodically (inversiondriving: see FIG. 46B). Since direct voltage can be prevented from beingapplied to a liquid crystal as much as possible by using this method,burn-in or the like caused by deterioration of the liquid crystalelement can be prevented. Note that a period of switching the polarity(an inversion period) may be the same as a period of rewriting voltage.In this case, generation of a flicker caused by inversion driving can bereduced since the inversion period is short. Further, the inversionperiod may be a period which is integral times the period of rewritingvoltage. In this case, power consumption can be reduced since theinversion period is long and frequency of writing voltage can bedecreased by changing the polarity.

FIG. 46C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 46A or 468 is applied to theliquid crystal element. Here, the voltage |V₁| is applied to the liquidcrystal element and transmittance of the liquid crystal element afterenough time passes corresponds to TR₁. Similarly, the voltage |V₂| isapplied to the liquid crystal element and transmittance of the liquidcrystal element after enough time passes corresponds to TR₂. Similarly,the voltage |V₃| is applied to the liquid crystal element andtransmittance of the liquid crystal element after enough time passescorresponds to TR₃. When the voltage applied to the liquid crystalelement is changed from |V₁| to |V₃| at the time of t₁, transmittance ofthe liquid crystal element is tried to be changed to TR₃ for severalframes as shown by a dashed line 30501. However, application of thevoltage |V₃| is terminated at the time of t₂, and the voltage |V₂| isapplied after the time of t₂. Therefore, transmittance of the liquidcrystal element does not become as shown by the dashed line 30501 butbecomes as shown by a solid line 30502. Here, it is preferable that avalue of the voltage |V₃| be set so that transmittance is approximatelyTR₂ at the time of t₂. Here, the voltage |V₃| is also referred to asoverdriving voltage.

The response time of the liquid crystal element can be controlled tosome extent by changing |V₃|, which is the overdriving voltage. This isbecause the response time of the liquid crystal element is changed bystrength of an electric field. Specifically, the response time of theliquid crystal element becomes shorter as the electric field isstronger, and the response time of the liquid crystal element becomeslonger as the electric field is weaker.

It is preferable that |V₃|, which is the overdriving voltage, be changedin accordance with the amount of change in the voltage, that is, thevoltage |V₁| and the voltage |V₂| which provide intended transmittanceTR₁ and TR₂. This is because appropriate response time can be alwaysobtained by changing |V₃|, which is the overdriving voltage, inaccordance with change in the response time of the liquid crystalelement even when the response time of the liquid crystal element ischanged by the amount of change in the voltage.

It is preferable that |V₃|, which is the overdriving voltage, be changeddepending on a mode of the liquid crystal element, such as a TN-mode, aVA-mode, an IPS-mode, or an OCB-mode. This is because appropriateresponse time can be always obtained by changing |V₃|, which is theoverdriving voltage, in accordance with change in the response time ofthe liquid crystal element even when the response time of the liquidcrystal element is changed depending on the mode of the liquid crystalelement.

The voltage rewriting period F may be the same as a frame period of aninput signal. In this case, a liquid crystal display device with lowmanufacturing cost can be obtained since a peripheral driver circuit ofthe liquid crystal display device can be simplified.

The voltage rewriting period F may be shorter than the frame period ofthe input signal. For example, the voltage rewriting period F may be onehalf the frame period of the input signal, or one third or less theframe period of the input signal. It is effective to combine this methodwith a measure against deterioration in quality of a moving image causedby hold driving of the liquid crystal display device, such as black datainsertion driving, backlight blinking, backlight scanning, orintermediate image insertion driving by motion compensation. That is,since required response time of the liquid crystal element is short inthe measure against deterioration in quality of a moving image caused byhold driving of the liquid crystal display device, the response time ofthe liquid crystal element can be relatively shortened easily by usingthe overdriving method described in this embodiment mode. Although theresponse time of the liquid crystal element can be essentially shortenedby a cell gap, a liquid crystal material, a mode of the liquid crystalelement, or the like, it is technically difficult to shorten theresponse time of the liquid crystal element. Therefore, it is veryimportant to use a method for shortening the response time of the liquidcrystal element by a driving method, such as overdriving.

The voltage rewriting period F may be longer than the frame period ofthe input signal. For example, the voltage rewriting period F may betwice the frame period of the input signal, or three times or more theframe period of the input signal. It is effective to combine this methodwith a means (a circuit) which determines whether voltage is notrewritten for a long period or not. That is, when the voltage is notrewritten for a long period, an operation of the circuit can be stoppedduring a period where no voltage is rewritten without performing arewriting operation of the voltage. Therefore, a liquid crystal displaydevice with low power consumption can be obtained.

Next, a specific method for changing the overdriving voltage |V₃| inaccordance with the voltage |V₁| and the voltage |V₂|, which provideintended transmittance TR₁ and TR₂, is described.

Since an overdriving circuit corresponds to a circuit for appropriatelycontrolling the overdriving voltage |V₃| in accordance with the voltage|V₁| and the voltage |V₂|, which provide intended transmittance TR₁ andTR₂, signals input to the overdriving circuit are a signal related tothe voltage |V₁|, which provides intended transmittance TR₁, and asignal related to the voltage |V₂|, which provides intendedtransmittance TR₂; and a signal output from the overdriving circuit is asignal related to the overdriving voltage |V₃ 1. Here, each of thesesignals may have an analog voltage value such as the voltage applied tothe liquid crystal element (e.g., |V₁₁, |V₂|, or |V₃|) or may be adigital signal for supplying the voltage applied to the liquid crystalelement. Here, the signal related to the overdriving circuit isdescribed as a digital signal.

First, a general structure of the overdriving circuit is described withreference to FIG. 82A. Here, input image signals 30101 a and 30101 b areused as signals for controlling the overdriving voltage. As a result ofprocessing these signals, an output image signal 30104 is to be outputas a signal which provides the overdriving voltage.

Since the voltage |V₁| and the voltage |V₂|, which provide intendedtransmittance TR₁ and TR₂, are image signals in adjacent frames, it ispreferable that the input image signals 30101 a and 301016 be also imagesignals in adjacent frames. In order to obtain such signals, the inputimage signal 30101 a is input to a delay circuit 30102 in FIG. 82A and asignal which is consequently output can be used as the input imagesignal 30101 b. For example, a memory can be given as the delay circuit30102. That is, the input image signal 30101 a is stored in the memoryin order to delay the input image signal 30101 a for one frame, and atthe same time, a signal stored in the previous frame is extracted fromthe memory as the input image signal 30101 b, and the input image signal30101 a and the input image signal 30101 b are simultaneously input to acorrection circuit 30103. Therefore, the image signals in adjacentframes can be handled. By inputting the image signals in adjacent framesto the correction circuit 30103, the output image signal 30104 can beobtained. Note that when a memory is used as the delay circuit 30102, amemory having capacity for storing an image signal for one frame inorder to delay the input image signal 30101 a for one frame (i.e., aframe memory) can be obtained. Thus, the memory can have a function as adelay circuit without causing excess and deficiency of memory capacity.

Next, the delay circuit 30102 formed mainly for reducing memory capacityis described. Since memory capacity can be reduced by using such acircuit as the delay circuit 30102, manufacturing cost can be reduced.

Specifically, a delay circuit as shown in FIG. 82B can be used as thedelay circuit 30102 having such characteristics. The delay circuit shownin FIG. 82B includes an encoder 30105, a memory 30106, and a decoder30107.

Operations of the delay circuit 30102 shown in FIG. 82B are as follows.First, compression processing is performed by the encoder 30105 beforethe input image signal 30101 a is stored in the memory 30106. Thus, sizeof data to be stored in the memory 30106 can be reduced. Accordingly,memory capacity can be reduced, and manufacturing cost can be reduced.Then, a compressed image signal is transferred to the decoder 30107 andextension processing is performed here. Thus, the signal which has beencompressed by the encoder 30105 can be restored. Here, compression andextension processing which is performed by the encoder 30105 and thedecoder 30107 may be reversible processing. Accordingly, since the imagesignal does not deteriorate even after compression and extensionprocessing is performed, memory capacity can be reduced without causingdeterioration of quality of an image, which is finally displayed on adevice. Further, compression and extension processing which is performedby the encoder 30105 and the decoder 30107 may be non-reversibleprocessing. Accordingly, since size of data of the compressed imagesignal can be made extremely small, memory capacity can be significantlyreduced.

As a method for reducing memory capacity, various methods can be used aswell as the above-described method. For example, a method in which colorinformation included in an image signal is reduced (e.g., tone reductionfrom 260 thousand colors to 65 thousand colors is performed) or theamount of data is reduced (resolution is reduced) without performingimage compression by an encoder can be used.

Next, specific examples of the correction circuit 30103 are describedwith reference to FIGS. 88C to 88E. The correction circuit 30103corresponds to a circuit for outputting an output image signal of acertain value from two input image signals. Here, when a relationbetween the two input image signals and the output image signal isnon-linear and it is difficult to calculate the relation by simpleoperation, a look up table (LUT) may be used as the correction circuit30103. Since the relation between the two input image signals and theoutput image signal is calculated in advance by measurement in a LUT,the output image signal corresponding to the two input image signals canbe calculated only by seeing the LUT (see FIG. 82C). By using a LUT30108 as the correction circuit 30103, the correction circuit 30103 canbe realized without complicated circuit design or the like.

Since the LUT 30108 is one of memories, it is preferable to reducememory capacity as much as possible in order to reduce manufacturingcost. As an example of the correction circuit 30103 for realizingreduction in memory capacity, a circuit shown in FIG. 82D can beconsidered. The correction circuit 30103 shown in FIG. 82D includes aLUT 30109 and an adder 30110. Difference data between the input imagesignal 30101 a and the output image signal 30104 to be output is storedin the LUT 30109. That is, corresponding difference data from the inputimage signal 30101 a and the input image signal 30101 b is extractedfrom the LUT 30109, and the extracted difference data and the inputimage signal 30101 a are added by the adder 30110, so that the outputimage signal 30104 can be obtained. Note that when data stored in theLUT 30109 is difference data, memory capacity of the LUT 30109 can bereduced. This is because data size of difference data is smaller thanthat of the output image signal 30104 as it is, so that memory capacitynecessary for the LUT 30109 can be reduced.

In addition, when the output image signal can be calculated by simpleoperation such as four arithmetic operations of the two input imagesignals, the correction circuit 30103 can be realized by combination ofsimple circuits such as an adder, a subtractor, and a multiplier.Accordingly, it is not necessary to use an LUT, and manufacturing costcan be significantly reduced. As such a circuit, a circuit shown in FIG.82E can be considered. The correction circuit 30103 shown in FIG. 82Eincludes a subtractor 30111, a multiplier 30112, and an adder 30113.First, difference between the input image signal 30101 a and the inputimage signal 30101 b is calculated by the subtractor 30111, After that,a differential value is multiplied by an appropriate coefficient byusing the multiplier 30112. Then, the differential value multiplied bythe appropriate coefficient is added to the input image signal 30101 aby the adder 30113; thus, the output image signal 30104 can be obtained.By using such a circuit, it is not necessary to use the LUT. Therefore,manufacturing cost can be significantly reduced.

By using the correction circuit 30103 shown in FIG. 82E under a certaincondition, output of the inappropriate output image signal 30104 can beprevented. The condition is that a differential value between the outputimage signal 30104 applying the overdriving voltage and the input imagesignals 30101 a and 30101 b has linearity. Inclination of this linearitycorresponds to a coefficient to be multiplied by using the adder 30112.That is, it is preferable that the correction circuit 30103 shown inFIG. 82E be used for a liquid crystal element having such properties. Asa liquid crystal element having such properties, an IPS-mode liquidcrystal element in which response time has little gray-scale dependencyis considered. For example, when the correction circuit 30103 shown inFIG. 82E is used for an IPS mode liquid crystal element in this manner,manufacturing cost can be significantly reduced and an overdrivingcircuit which can prevent output of the inappropriate output imagesignal 30104 can be obtained.

Operations which are similar to those of the circuit shown in FIGS. 82Ato 82E may be realized by software processing. As the memory used forthe delay circuit, another memory included in the liquid crystal displaydevice, a memory included in a device which transfers an image displayedon the liquid crystal display device (e.g., a video card or the likeincluded in a personal computer or a device similar to the personalcomputer) can be used. Accordingly, not only can manufacturing cost bereduced, intensity of overdriving, availability, or the like can beselected in accordance with user's preference.

Next, driving which controls a potential of a common line is describedwith reference to FIGS. 83A and 83B. FIG. 83A shows a plurality of pixelcircuits in which one common line is provided with respect to one scanline in a display device using a display element which has capacitiveproperties, such as a liquid crystal element. Each of the pixel circuitsshown in FIG. 83A includes a transistor 30201, an auxiliary capacitor30202, a display element 30203, a video signal line 30204, a scan line30205, and a common line 30206.

A gate electrode of the transistor 30201 is electrically connected tothe scan line 30205, one of a source electrode and a drain electrode ofthe transistor 30201 is electrically connected to the video signal line30204, and the other of the source electrode and the drain electrode ofthe transistor 30201 is electrically connected to one electrode of theauxiliary capacitor 30202 and one electrode of the display element30203. The other electrode of the auxiliary capacitor 30202 iselectrically connected to the common line 30206.

First, in each of pixels selected by the scan line 30205, voltagecorresponding to a video signal is applied to the display element 30203and the auxiliary capacitor 30202 through the video signal line 30204since the transistor 30201 is turned on. At this time, when the videosignal is a signal which makes all of pixels connected to the commonline 30206 display a minimum gray scale or a maximum gray scale, it isnot necessary that the video signal be written in each of the pixelsthrough the video signal line 30204. Voltage applied to the displayelement 30203 can be changed by changing a potential of the common line30206 instead of writing the video signal through the video signal line30204.

Next, FIG. 83B shows diagram showing a plurality of pixel circuits inwhich two common lines are provided with respect to one scan line in adisplay device using a display element which has capacitive properties,such as a Liquid crystal element. Each of the pixel circuits shown inFIG. 83B includes a transistor 30211, an auxiliary capacitor 30212, adisplay element 30213, a video signal line 30214, a scan line 30215, afirst common line 30216, and a second common line 30217.

A gate electrode of the transistor 30211 is electrically connected tothe scan line 30215, one of a source electrode and a drain electrode ofthe transistor 30211 is electrically connected to the video signal line30214, and the other of the source electrode and the drain electrode ofthe transistor 30211 is electrically connected to one electrode of theauxiliary capacitor 30212 and one electrode of the display element30213. The other electrode of the auxiliary capacitor 30212 iselectrically connected to the first common line 30216. Further, in apixel which is adjacent to the pixel, the other electrode of theauxiliary capacitor 30212 is electrically connected to the second commonline 30217.

In the pixel circuits shown in FIG. 83B, the number of pixels which areelectrically connected to one common line is small. Accordingly, bychanging a potential of the first common line 30216 or the second commonline 30217 instead of writing a video signal through the video signalline 30214, frequency of changing voltage applied to the display element30213 is significantly increased. In addition, source inversion drivingor dot inversion driving can be performed. By performing sourceinversion driving or dot inversion driving, reliability of the elementcan be improved and a flicker can be suppressed.

Next, a scanning backlight is described with reference to FIGS. 84A to84C. FIG. 84A shows a scanning backlight in which cold cathodefluorescent lamps are arranged. The scanning backlight shown in FIG. 84Aincludes a diffusion plate 30301 and N pieces of cold cathodefluorescent lamps 30302-1 to 30302-N. The N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N are arranged on the back side ofthe diffusion plate 30301, so that the N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N can be scanned while luminancethereof is changed.

Change in luminance of each of the cold cathode fluorescent lamps inscanning is described with reference to FIG. 84C. First, luminance ofthe cold cathode fluorescent lamp 30302-1 is changed for a certainperiod. After that, luminance of the cold cathode fluorescent lamp30302-2 which is provided adjacent to the cold cathode fluorescent lamp30302-1 is changed for the same period. In this manner, luminance ischanged sequentially from the cold cathode fluorescent lamps 30302-1 to30302-N. Note that although luminance which is changed for a certainperiod is set to be lower than original luminance in FIG. 84C, it may behigher than original luminance. In addition, although scanning isperformed from the cold cathode fluorescent lamps 30302-1 to 30302-N,scanning may be performed from the cold cathode fluorescent lamps30302-N to 30302-1, which is in a reversed order.

By performing driving as in FIGS. 84A to 84C, average luminance of thebacklight can be decreased. Therefore, power consumption of thebacklight, which mainly takes up power consumption of the liquid crystaldisplay device, can be reduced.

Note that an LED may be used as a light source of the scanningbacklight. FIG. 84B shows the scanning backlight in that case. Thescanning backlight shown in FIG. 84B includes a diffusion plate 30311and light sources 30312-1 to 30312-N, in each of which LEDs arearranged. When the LED is used as the light source of the scanningbacklight, it is advantageous in that the backlight can be thin andlightweight and that a color reproduction area can be widened. Further,since the LEDs which are arranged in each of the light sources 30312-1to 30312-N can be similarly scanned, a dot scanning backlight can alsobe obtained. By using the dot scanning backlight, image quality of amoving image can be further improved.

When the LED is used as the light source of the backlight, driving canbe performed by changing luminance as shown in FIG. 84C as well.

Next, high frequency driving is described with reference to FIGS. 85Aand 85B. FIG. 85A is a view in which one image and one intermediateimage are displayed in one frame period 30400. Reference numeral 30401denotes an image of the frame; 30402 denotes an intermediate image ofthe frame; 30403 denotes an image of the next frame; and 30404 denotesan intermediate image of the next frame.

The intermediate image 30402 of the frame may be an image which is madebased on video signals of the frame and the next frame. Alternatively,the intermediate image 30402 of the frame may be an image which is madefrom the image 30401 of the frame. Further alternatively, theintermediate image 30402 of the frame may be a black image. Thus, imagequality of a moving image of a hold-type display device can be improved.When one image and one intermediate image are displayed in the one frameperiod 30400, there is an advantage in that consistency with a framerate of the video signal can be easily obtained and an image processingcircuit is not complicated.

FIG. 85B is a view in which one image and two intermediate images aredisplayed in a period with two successive one frame periods 30400 (i.e.,two frame periods). Reference numeral 30411 denotes an image of theframe; 30412 denotes an intermediate image of the frame; 30413 denotesan intermediate image of the next frame; and 30414 denotes an image of aframe after next.

Each of the intermediate image 30412 of the frame and the intermediateimage 30413 of the next frame may be an image which is made based onvideo signals of the frame, the next frame, and the frame after next.Alternatively, each of the intermediate image 30412 of the frame and theintermediate image 30413 of the next frame may be a black image. Whenone image and two intermediate images are displayed in the two frameperiods, there is an advantage in that operating frequency of aperipheral driver circuit is not so high and image quality of a movingimage can be effectively improved.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 11

In this embodiment mode, a peripheral portion of a liquid crystal panelis described.

FIG. 49 shows an example of a liquid crystal display device including aso-called edge-light type backlight unit 20101 and a liquid crystalpanel 20107. An edge-light type corresponds to a type in which a lightsource is provided at an end of a backlight unit and fluorescence of thelight source is emitted from the entire light-emitting surface. Theedge-light type backlight unit is thin and can save power.

The backlight unit 20101 includes a diffusion plate 20102, a light guideplate 20103, a reflection plate 20104, a lamp reflector 20105, and alight source 20106.

The light source 20106 has a function to emit light as necessary. Forexample, as the light source 20106, a cold cathode fluorescent lamp, ahot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like can be used.

FIGS. 50A to 50D each show a detailed structure of the edge-light typebacklight unit. Note that description of a diffusion plate, a lightguide plate, a reflection plate, and the like is omitted.

A backlight unit 20201 shown in FIG. 50A has a structure in which a coldcathode fluorescent lamp 20203 is used as a light source. A lampreflector 20202 is provided to efficiently reflect light from the coldcathode fluorescent lamp 20203. Such a structure is often used for alarge display device because luminance from the cold cathode fluorescentlamp 20203 is high.

A backlight unit 20211 shown in FIG. 50B has a structure in whichlight-emitting diodes (LEDs) 20213 are used as light sources. Forexample, the light-emitting diodes (LEDs) 20213 which emit white lightare provided at a predetermined interval. Further, a lamp reflector20212 is provided to efficiently reflect light from the light-emittingdiodes (LEDs) 20213.

A backlight unit 20221 shown in FIG. 50C has a structure in whichlight-emitting diodes (LEDs) 20223, light-emitting diodes (LEDs) 20224,and light-emitting diodes (LEDs) 20225 of each color of RGB are used aslight sources. The light-emitting diodes (LEDs) 20223, thelight-emitting diodes (LEDs) 20224, and the light-emitting diodes (LEDs)20225 of each color of RGB are each provided at a predeterminedinterval. By using the light-emitting diodes (LEDs) 20223, 20224, and20225 of each color of RGB, color reproducibility can be improved. Inaddition, a lamp reflector 20222 is provided to efficiently reflectlight from the light-emitting diodes.

A backlight unit 20231 shown in FIG. 50D has a structure in whichlight-emitting diodes (LEDs) 20233, light-emitting diodes (LEDs) 20234,and light-emitting diodes (LEDs) 20235 of each color of RGB are used aslight sources. For example, among the light-emitting diodes (LEDs)20233, the light-emitting diodes (LEDs) 20234, and the light-emittingdiodes (LEDs) 20235 of each color of RGB, the light-emitting diodes of acolor with low emission intensity (e.g., green) are provided more thanother light-emitting diodes. By using the light-emitting diodes (LEDs)20233, 20234, and 20235 of each color of RGB, color reproducibility canbe improved. In addition, a lamp reflector 20232 is provided toefficiently reflect light from the light-emitting diodes.

FIG. 53 shows an example of a liquid crystal display device including aso-called direct-type backlight unit and a liquid crystal panel. Adirect type corresponds to a type in which a light source is provideddirectly under a light-emitting surface and fluorescence of the lightsource is emitted from the entire light-emitting surface. Thedirect-type backlight unit can efficiently utilize the amount of emittedlight.

A backlight unit 20500 includes a diffusion plate 20501, alight-shielding plate 20502, a lamp reflector 20503, a light source20504, and a liquid crystal panel 20505.

The light source 20504 has a function to emit light as necessary. Forexample, as the light source 20504, a cold cathode fluorescent lamp, ahot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like can be used.

FIG. 51 shows an example of a structure of a polarizing plate (alsoreferred to as a polarizing film).

A polarizing film 20300 includes a protective film 20301, a substratefilm 20302, a PVA polarizing film 20303, a substrate film 20304, anadhesive layer 20305, and a mold release film 20306.

When the PVA polarizing film 20303 is interposed between films (thesubstrate film 20302 and the substrate film 20304) to be base materials,reliability can be improved. Note that the PVA polarizing film 20303 maybe interposed by triacetyl cellulose (TAC) films with highlight-transmitting properties and high durability. Note also that thesubstrate films and the TAC films each function as a protective film ofa polarizer included in the PVA polarizing film 20303.

The adhesive layer 20305 which is to be attached to a glass substrate ofthe liquid crystal panel is attached to one of the substrate films (thesubstrate film 20304). Note that the adhesive layer 20305 is formed byapplying an adhesive to one of the substrate films (the substrate film20304). The adhesive layer 20305 is provided with the mold release film20306 (a separate film).

The other of the substrates films (the substrate film 20302) is providedwith the protective film 20301.

A hard coating scattering layer (an anti-glare layer) may be provided ona surface of the polarizing film 20300. Since the surface of the hardcoating scattering layer has minute unevenness formed by AG treatmentand has an anti-glare function which scatters external light, reflectionof external light in the liquid crystal panel and surface reflection canbe prevented.

A treatment in which a plurality of optical thin film layers havingdifferent refractive indexes are layered (also referred to asanti-reflection treatment or AR treatment) may be performed on thesurface of the polarizing film 20300. The plurality of layered opticalthin film layers having different refractive indexes can reducereflectivity on the surface by an interference effect of light.

FIGS. 52A to 52C show examples of a system block of a liquid crystaldisplay device.

In a pixel portion 20405, signal lines 20412 which are extended from asignal line driver circuit 20403 are provided. In the pixel portion20405, scan lines 20410 which are extended from a scan line drivercircuit 20404 are also provided. Further, a plurality of pixels arearranged in matrix in cross regions of the signal lines 20412 and thescan lines 20410. Note that each of the plurality of pixels includes aswitching element. Therefore, voltage for controlling inclination ofliquid crystal molecules can be separately input to each of theplurality of pixels. A structure in which a switching element isprovided in each cross region in this manner is referred to as an activematrix type. Note that the invention is not limited to such an activematrix type, and a structure of a passive matrix type may be used. In apassive matrix type, a switching element is not included in each pixel,so that a process is simple.

A driver circuit portion 20408 includes a control circuit 20402, thesignal line driver circuit 20403, and the scan line driver circuit20404. An image signal 20401 is input to the control circuit 20402. Thesignal line driver circuit 20403 and the scan line driver circuit 20404are controlled by the control circuit 20402 in accordance with thisimage signal 20401. The control circuit 20402 inputs a control signal toeach of the signal line driver circuit 20403 and the scan line drivercircuit 20404. Then, in accordance with the control signal, the signalline driver circuit 20403 inputs a video signal to each of the signallines 20412 and the scan line driver circuit 20404 inputs a scan signalto each of the scan lines 20410. Then, the switching element included inthe pixel is selected in accordance with the scan signal, and the videosignal is input to a pixel electrode of the pixel.

The control circuit 20402 also controls a power supply 20407 inaccordance with the image signal 20401. The power supply 20407 includesa means to supply power to a lighting unit 20406. As the lighting unit20406, an edge-light type backlight unit or a direct-type backlight unitcan be used. Note that a front light may be used as the lighting unit20406. A front light corresponds to a plate-like lighting unit includinga luminous body and a light conducting body, which is attached to thefront surface side of a pixel portion and illuminates the whole area. Byusing such a lighting unit, the pixel portion can be uniformlyilluminated at low power consumption.

As shown in FIG. 52B, the scan line driver circuit 20404 includes ashift register 20441, a level shifter 20442, and a circuit functioningas a buffer 20443. A signal such as a gate start pulse (GSP) or a gateclock signal (GCK) is input to the shift register 20441.

As shown in FIG. 52C, the signal line driver circuit 20403 includes ashift register 20431, a first latch 20432, a second latch 20433, a levelshifter 20434, and a circuit functioning as a buffer 20435. The circuitfunctioning as the buffer 20435 corresponds to a circuit which has afunction to amplify a weak signal and includes an operational amplifieror the like. A signal such as a start pulse (SSP) is input to the levelshifter 20434, and data (DATA) such as a video signal is input to thefirst latch 20432. A latch (LAT) signal can be temporally held in thesecond latch 20433 and is simultaneously input to the pixel portion20405. This is referred to as line sequential driving. Therefore, when apixel in which not line sequential driving but dot sequential driving isperformed is employed, the second latch can be omitted.

In this embodiment mode, various types of liquid crystal panels can beused. For example, a structure in which a liquid crystal layer is sealedbetween two substrates can be used for the liquid crystal panel. Atransistor, a capacitor, a pixel electrode, an alignment film, or thelike is formed over one substrate. A polarizing plate, a retardationplate, or a prism sheet may be provided on the surface opposite to a topsurface of one substrate. A color filter, a black matrix, an oppositeelectrode, an alignment film, or the like is provided on the othersubstrate. A polarizing plate or a retardation plate may be provided onthe surface opposite to a top surface of the other substrate. Note thatthe color filter and the black matrix may be formed over the top surfaceof one substrate. In addition, three-dimensional display can beperformed by providing a slit (a grid) on the top surface or the surfaceopposite to the top surface of one substrate.

Each of the polarizing plate, the retardation plate, and the prism sheetcan be provided between the two substrates. Alternatively, each of thepolarizing plate, the retardation plate, and the prism sheet can beintegrated with one of the two substrates.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 12

In this embodiment mode, a structure and an operation of a pixel whichcan be applied to a liquid crystal display device are described.

In this embodiment mode, as an operation mode of a liquid crystalelement, a TN (Twisted Nematic) mode, an IPS (In-Plane-Switching) mode,an FFS (Fringe Field Switching) mode, an MVA (Multi-domain VerticalAlignment) mode, a PVA (Patterned Vertical Alignment) mode, an ASM(Axially Symmetric aligned Microcell) mode, an OCB (Optical CompensatedBirefringence) mode, an FLC (Ferroelectric Liquid Crystal) mode, an AFLC(AntiFerroelectric Liquid Crystal) mode, or the like can be used.

FIG. 54A shows an example of a pixel structure which can be applied tothe liquid crystal display device.

A pixel 40100 includes a transistor 40101, a liquid crystal element40102, and a capacitor 40103. A gate of the transistor 40101 isconnected to a wiring 40105. A first terminal of the transistor 40101 isconnected to a wiring 40104. A second terminal of the transistor 40101is connected to a first electrode of the liquid crystal element 40102and a first electrode of the capacitor 40103. A second electrode of theliquid crystal element 40102 corresponds to an opposite electrode 40107.A second electrode of the capacitor 40103 is connected to a wiring40106.

The wiring 40104 functions as a signal line. The wiring 40105 functionsas a scan line. The wiring 40106 functions as a capacitor line. Thetransistor 40101 functions as a switch. The capacitor 40103 functions asa storage capacitor.

It is only necessary that the transistor 40101 function as a switch. Thetransistor 40101 may be a p-channel transistor or an n-channeltransistor.

FIG. 54B shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 54B shows anexample of a pixel structure which can be applied to a liquid crystaldisplay device suitable for a lateral electric field mode (including anIPS mode and an FFS mode).

A pixel 40110 includes a transistor 40111, a liquid crystal element40112, and a capacitor 40113. A gate of the transistor 40111 isconnected to a wiring 40115. A first terminal of the transistor 40111 isconnected to a wiring 40114. A second terminal of the transistor 40111is connected to a first electrode of the liquid crystal element 40112and a first electrode of the capacitor 40113. A second electrode of theliquid crystal element 40112 is connected to a wiring 40116. A secondelectrode of the capacitor 40103 is connected to the wiring 40116.

The wiring 40114 functions as a signal line. The wiring 40115 functionsas a scan line. The wiring 40116 functions as a capacitor line. Thetransistor 40111 functions as a switch. The capacitor 40113 functions asa storage capacitor.

It is only necessary that the transistor 40111 function as a switch. Thetransistor 40111 may be a p-channel transistor or an n-channeltransistor.

FIG. 55 shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 55 shows anexample of a pixel structure in which an aperture ratio of a pixel canbe increased by reducing the number of wirings.

FIG. 55 shows two pixels (a pixel 40200 and a pixel 40210) which areprovided in the same column direction. For example, when the pixel 40200is provided in an N-th row, the pixel 40210 is provided in an (N+1)throw.

The pixel 40200 includes a transistor 40201, a liquid crystal element40202, and a capacitor 40203. A gate of the transistor 40201 isconnected to a wiring 40205. A first terminal of the transistor 40201 isconnected to a wiring 40204. A second terminal of the transistor 40201is connected to a first electrode of the liquid crystal element 40202and a first electrode of the capacitor 40203. A second electrode of theliquid crystal element 40202 corresponds to an opposite electrode 40207.A second electrode of the capacitor 40203 is connected to a wiring whichis the same as that connected to a gate of a transistor in the previousrow.

The pixel 40210 includes a transistor 40211, a liquid crystal element40212, and a capacitor 40213. A gate of the transistor 40211 isconnected to a wiring 40215. A first terminal of the transistor 40211 isconnected to the wiring 40204. A second terminal of the transistor 40211is connected to a first electrode of the liquid crystal element 40212and a first electrode of the capacitor 40213. A second electrode of theliquid crystal element 40212 corresponds to an opposite electrode 40217.A second electrode of the capacitor 40213 is connected to a wiring whichis the same as that connected to the gate of the transistor in theprevious row (i.e., the wiring 40205).

The wiring 40204 functions as a signal line. The wiring 40205 functionsas a scan line of the N-th row, and also as a capacitor line of the(N+1)th row. The transistor 40201 functions as a switch. The capacitor40203 functions as a storage capacitor.

The wiring 40215 functions as a scan line of the (N+1)th row, and alsoas a capacitor line of an (N+2)th row. The transistor 40211 functions asa switch. The capacitor 40213 functions as a storage capacitor.

It is only necessary that each of the transistor 40201 and thetransistor 40211 function as a switch. Each of the transistor 40201 andthe transistor 40211 may be a p-channel transistor or an n-channeltransistor.

FIG. 56 shows an example of a pixel structure which can be applied tothe liquid crystal display device. In particular, FIG. 56 shows anexample of a pixel structure in which a viewing angle can be improved byusing a subpixel.

A pixel 40320 includes a subpixel 40300 and a subpixel 40310. Althoughthe case where the pixel 40320 includes two subpixels is describedbelow, the pixel 40320 may include three or more subpixels.

The subpixel 40300 includes a transistor 40301, a liquid crystal element40302, and a capacitor 40303. A gate of the transistor 40301 isconnected to a wiring 40305. A first terminal of the transistor 40301 isconnected to a wiring 40304. A second terminal of the transistor 40301is connected to a first electrode of the liquid crystal element 40302and a first electrode of the capacitor 40303. A second electrode of theliquid crystal element 40302 corresponds to an opposite electrode 40307.A second electrode of the capacitor 40303 is connected to a wiring40306.

The subpixel 40310 includes a transistor 40311, a liquid crystal element40312, and a capacitor 40313. A gate of the transistor 40311 isconnected to a wiring 40315. A first terminal of the transistor 40311 isconnected to the wiring 40304. A second terminal of the transistor 40311is connected to a first electrode of the liquid crystal element 40312and a first electrode of the capacitor 40313. A second electrode of theliquid crystal element 40312 corresponds to an opposite electrode 40317.A second electrode of the capacitor 40313 is connected to the wiring40306.

The wiring 40304 functions as a signal line. The wiring 40305 functionsas a scan line. The wiring 40315 functions as a signal line. The wiring40306 functions as a capacitor line. The transistor 40301 functions as aswitch. The transistor 40311 functions as a switch. The capacitor 40303functions as a storage capacitor. The capacitor 40313 functions as astorage capacitor.

It is only necessary that the transistor 40301 function as a switch. Thetransistor 40301 may be a p-channel transistor or an n-channeltransistor. It is only necessary that the transistor 40311 function as aswitch. The transistor 40311 may be a p-channel transistor or ann-channel transistor.

A video signal input to the subpixel 40300 may be a value which isdifferent from that of a video signal input to the subpixel 40310. Inthis case; the viewing angle can be widened because alignment of liquidcrystal molecules of the liquid crystal element 40302 is different fromalignment of liquid crystal molecules of the liquid crystal element40312.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detailed, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 13

In this embodiment mode, various liquid crystal modes are described.

First, various liquid crystal modes are described with reference tocross-sectional views.

FIGS. 57A and 57B are schematic views of cross sections of a TN mode.

A liquid crystal layer 50100 is held between a first substrate 50101 anda second substrate 50102 which are provided so as to be opposite to eachother. A first electrode 50105 is formed on a top surface of the firstsubstrate 50101. A second electrode 50106 is formed on a top surface ofthe second substrate 50102. A first polarizing plate 50103 is providedon a surface of the first substrate 50101, which does not face theliquid crystal layer 50100. A second polarizing plate 50104 is providedon a surface of the second substrate 50102, which does not face theliquid crystal layer 50100. Note that the first polarizing plate 50103and the second polarizing plate 50104 are provided so as to be in across nicol state.

The first polarizing plate 50103 may be provided on the top surface ofthe first substrate 50101, that is, may be provided between the firstsubstrate 50101 and the liquid crystal layer 50100. The secondpolarizing plate 50104 may be provided on the top surface of the secondsubstrate 50102, that is, may be provided between the second substrate50102 and the liquid crystal layer 50100.

It is only necessary that at least one of the first electrode 50105 andthe second electrode 50106 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50105 and the second electrode50106 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 57A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50105 and the second electrode50106 (referred to as a vertical electric field mode).

FIG. 57B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50105 and the secondelectrode 50106.

FIGS. 58A and 58B are schematic views of cross sections of a VA mode. Inthe VA mode, liquid crystal molecules are aligned such that they arevertical to a substrate when there is no electric field.

A liquid crystal layer 50200 is held between a first substrate 50201 anda second substrate 50202 which are provided so as to be opposite to eachother. A first electrode 50205 is formed on a top surface of the firstsubstrate 50201. A second electrode 50206 is formed on a top surface ofthe second substrate 50202.

A first polarizing plate 50203 is provided on a surface of the firstsubstrate 50201, which does not face the liquid crystal layer. A secondpolarizing plate 50204 is provided on a surface of the second substrate50202, which does not face the liquid crystal layer. Note that the firstpolarizing plate 50203 and the second polarizing plate 50204 areprovided so as to be in a cross nicol state.

The first polarizing plate 50203 may be provided on the top surface ofthe first substrate 50201, that is, may be provided between the firstsubstrate 50201 and the liquid crystal layer. The second polarizingplate 50204 may be provided on the top surface of the second substrate50202, that is, may be provided between the second substrate 50202 andthe liquid crystal layer 50200.

It is only necessary that at least one of the first electrode 50205 andthe second electrode 50206 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50205 and the second electrode50206 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 58A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50205 and the second electrode50206 (referred to as a vertical electric field mode).

FIG. 58B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50205 and the secondelectrode 50206.

FIGS. 58C and 58D are schematic views of cross sections of an MVA mode.In the MVA mode, viewing angle dependency of each portion is compensatedby each other.

A liquid crystal layer 50210 is held between a first substrate 50211 anda second substrate 50212 which are provided so as to be opposite to eachother. A first electrode 50215 is formed on a top surface of the firstsubstrate 50211. A second electrode 50216 is formed on a top surface ofthe second substrate 50212. A first projection 50217 for controllingalignment is formed on the first electrode 50215. A second projection50218 for controlling alignment is formed over the second electrode50216. A first polarizing plate 50213 is provided on a surface of thefirst substrate 50211, which does not face the liquid crystal layer50210. A second polarizing plate 50214 is provided on a surface of thesecond substrate 50212, which does not face the liquid crystal layer50210. Note that the first polarizing plate 50213 and the secondpolarizing plate 50214 are provided so as to be in a cross nicol state.

The first polarizing plate 50213 may be provided on the top surface ofthe to first substrate 50211, that is, may be provided between the firstsubstrate 50211 and the liquid crystal layer. The second polarizingplate 50214 may be provided on the top surface of the second substrate50212, that is, may be provided between the second substrate 50212 andthe liquid crystal layer.

It is only necessary that at least one of the first electrode 50215 andthe second electrode 50216 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50215 and the second electrode50216 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 58C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50215 and the second electrode50216 (referred to as a vertical electric field mode).

FIG. 58D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50215 and the secondelectrode 50216.

FIGS. 59A and 59B are schematic views of cross sections of an OCB mode.In the OCB mode, viewing angle dependency is low because alignment ofliquid crystal molecules in a liquid crystal layer can be opticallycompensated. This state of the liquid crystal molecules is referred toas bend alignment.

A liquid crystal layer 50300 is held between a first substrate 50301 anda second substrate 50302 which are provided so as to be opposite to eachother. A first electrode 50305 is formed on a top surface of the firstsubstrate 50301. A second electrode 50306 is formed on a top surface ofthe second substrate 50302. A first polarizing plate 50303 is providedon a surface of the first substrate 50301, which does not face theliquid crystal layer 50300. A second polarizing plate 50304 is providedon a surface of the second substrate 50302, which does not face theliquid crystal layer 50300. Note that the first polarizing plate 50303and the second polarizing plate 50304 are provided so as to be in across nicol state.

The first polarizing plate 50303 may be provided on the top surface ofthe first substrate 50301, that is, may be provided between the firstsubstrate 50301 and the liquid crystal layer 50300. The secondpolarizing plate 50304 may be provided on the top surface of the secondsubstrate 50302, that is, may be provided between the second substrate50302 and the liquid crystal layer 50300.

It is only necessary that at least one of the first electrode 50305 andthe second electrode 50306 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50305 and the second electrode50306 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 59A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50305 and the second electrode50306 (referred to as a vertical electric field mode).

FIG. 59B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50305 and the secondelectrode 50306.

FIGS. 59C and 59D are schematic views of cross sections of an FLC modeor an AFLC mode.

A liquid crystal layer 50310 is held between a first substrate 50311 anda second substrate 50312 which are provided so as to be opposite to eachother. A first electrode 50315 is formed on a top surface of the firstsubstrate 50311. A second electrode 50316 is formed on a top surface ofthe second substrate 50312. A first polarizing plate 50313 is providedon a surface of the first substrate 50311, which does not face theliquid crystal layer 50310. A second polarizing plate 50314 is providedon a surface of the second substrate 50312, which does not face theliquid crystal layer 50310. Note that the first polarizing plate 50313and the second polarizing plate 50314 are provided so as to be in across nicol state.

The first polarizing plate 50313 may be provided on the top surface ofthe first substrate 50311, that is, may be provided between the firstsubstrate 50311 and the liquid crystal layer 50310. The secondpolarizing plate 50314 may be provided on the top surface of the secondsubstrate 50312, that is, may be provided between the second substrate50312 and the liquid crystal layer 50310.

It is only necessary that at least one of the first electrode 50315 andthe second electrode 50316 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50315 and the second electrode50316 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 59C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50315 and the second electrode50316 (referred to as a vertical electric field mode).

FIG. 59D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50315 and the secondelectrode 50316.

FIGS. 60A and 60B are schematic views of cross sections of an IPS mode.In the IPS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50400 is held between a first substrate 50401 anda second substrate 50402 which are provided so as to be opposite to eachother. A first electrode 50405 and a second electrode 50406 are formedon a top surface of the second substrate 50402. A first polarizing plate50403 is provided on a surface of the first substrate 50401, which doesnot face the liquid crystal layer 50400. A second polarizing plate 50404is provided on a surface of the second substrate 50402, which does notface the liquid crystal layer 50400. Note that the first polarizingplate 50403 and the second polarizing plate 50404 are provided so as tobe in a cross nicol state.

The first polarizing plate 50403 may be provided on the top surface ofthe first substrate 50401, that is, may be provided between the firstsubstrate 50401 and the liquid crystal layer 50400. The secondpolarizing plate 50404 may be provided on the top surface of the secondsubstrate 50402, that is, may be provided between the second substrate50402 and the liquid crystal layer 50400.

It is only necessary that at least one of the first electrode 50405 andthe second electrode 50406 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50405 and the second electrode50406 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 60A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50405 and the second electrode50406 (referred to as a vertical electric field mode).

FIG. 60B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50405 and the secondelectrode 50406.

FIGS. 60C and 60D are schematic views of cross sections of an FFS mode.In the FFS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50410 is held between a first substrate 50411 anda second substrate 50412 which are provided so as to be opposite to eachother. A second electrode 50416 is formed on a top surface of the secondsubstrate 50412. An insulating film 50417 is formed on a top surface ofthe second electrode 50416. A first electrode 50415 is formed over theinsulating film 50417. A first polarizing plate 50413 is provided on asurface of the first substrate 50411, which does not face the liquidcrystal layer 50410. A second polarizing plate 50414 is provided on asurface of the second substrate 50412, which does not face the liquidcrystal layer 50410. Note that the first polarizing plate 50413 and thesecond polarizing plate 50414 are provided so as to be in a cross nicolstate.

The first polarizing plate 50413 may be provided on the top surface ofthe first substrate 50411, that is, may be provided between the firstsubstrate 50411 and the liquid crystal layer 50410. The secondpolarizing plate 50414 may be provided on the top surface of the secondsubstrate 50412, that is, may be provided between the second substrate50412 and the liquid crystal layer 50410.

It is only necessary that at least one of the first electrode 50415 andthe second electrode 50416 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50415 and the second electrode50416 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 60C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50415 and the second electrode50416 (referred to as a vertical electric field mode). FIG. 60D is aschematic view of a cross section in the case where voltage is notapplied to the first electrode 50415 and the second electrode 50416.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 14

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a liquid crystal displaydevice is described.

Pixel structures in the case where each liquid crystal mode and atransistor are combined are described with reference to cross-sectionalviews of pixels.

As the transistor, a thin film transistor (TFT) including a non-singlecrystalline semiconductor layer typified by amorphous silicon,polycrystalline silicon, microcrystalline (also referred to assemi-amorphous) silicon, or the like can be used.

As a structure of the transistor, a top-gate structure, a bottom-gatestructure, or the like can be used. Note that a channel-etchedtransistor, a channel-protective transistor, or the like can be used asa bottom-gate transistor.

FIG. 61 is an example of a cross-sectional view of a pixel in the casewhere a TN mode and a transistor are combined. A liquid crystal 10111having liquid crystal molecules 10118 is held between a first substrate10101 and a second substrate 10116. The first substrate 10101 isprovided with a transistor, a pixel electrode, an alignment film, andthe like. The second substrate 10116 is provided with a light-shieldingfilm 10114, a color filter 10115, an opposite electrode, an alignmentfilm, and the like. In addition, a spacer 10117 is provided between thefirst substrate 10101 and the second substrate 10116. By applying thepixel structure shown in FIG. 61 to a liquid crystal display device, aliquid crystal display device can be formed at low cost.

FIG. 62A is an example of a cross-sectional view of a pixel in the casewhere an MVA (Multi-domain Vertical Alignment) mode and a transistor arecombined. A liquid crystal 10211 having liquid crystal molecules 10218is held between a first substrate 10201 and a second substrate 10216.The first substrate 10201 is provided with a transistor, a pixelelectrode, an alignment film, and the like. The second substrate 10216is provided with a light-shielding film 10214, a color filter 10215, anopposite electrode, a projection 10219 for alignment control, analignment film, and the like. In addition, a spacer 10217 is providedbetween the first substrate 10201 and the second substrate 10216. Byapplying the pixel structure shown in FIG. 62A to a liquid crystaldisplay device, a liquid crystal display device having a wide viewingangle, high response speed, and high contrast can be obtained.

FIG. 62B is an example of a cross-sectional view of a pixel in the casewhere a PVA (Patterned Vertical Alignment) mode and a transistor arecombined. A liquid crystal 10241 having liquid crystal molecules 10248is held between a first substrate 10231 and a second substrate 10246.The first substrate 10231 is provided with a transistor, a pixelelectrode, an alignment film, and the like. The second substrate 10246is provided with a light-shielding film 10244, a color filter 10245, anopposite electrode, an alignment film, and the like. Note that the pixelelectrode includes an electrode notch portion 10249. In addition, aspacer 10247 is provided between the first substrate 10231 and thesecond substrate 10246. By applying the pixel structure shown in FIG.62B to a liquid crystal display device, a liquid crystal display devicehaving a wide viewing angle, high response speed, and high contrast canbe obtained.

FIG. 63A is an example of a cross-sectional view of a pixel in the casewhere an IPS (In-Plane-Switching) mode and a transistor are combined. Aliquid crystal 10311 having liquid crystal molecules 10318 is heldbetween a first substrate 10301 and a second substrate 10316. The firstsubstrate 10301 is provided with a transistor, a pixel electrode, acommon electrode, an alignment film, and the like. The second substrate10316 is provided with a light-shielding film 10314, a color filter10315, an alignment film, and the like. In addition, a spacer 10317 isprovided between the first substrate 10301 and the second substrate10316. By applying the pixel structure shown in FIG. 63A to a liquidcrystal display device, a liquid crystal display device having a wideviewing angle and response speed with low dependency on gray scale inprinciple can be obtained.

FIG. 63B is an example of a cross-sectional view of a pixel in the casewhere an FFS (Fringe Field Switching) mode and a transistor arecombined. A liquid crystal 10341 having liquid crystal molecules 10348is held between a first substrate 10331 and a second substrate 10346.The first substrate 10331 is provided with a transistor, a pixelelectrode, a common electrode, an alignment film, and the like. Thesecond substrate 10346 is provided with a light-shielding film 10344, acolor filter 10345, an alignment film, and the like. In addition, aspacer 10347 is provided between the first substrate 10331 and thesecond substrate 10346. By applying the pixel structure shown in FIG.63B to a liquid crystal display device, a liquid crystal display devicehaving a wide viewing angle and response speed with low dependency ongray scale in principle can be obtained.

Here, materials which can be used for conductive layers or insulatingfilms are described.

As a first insulating film 10102 in FIG. 61, a first insulating film10202 in FIG. 62A, a first insulating film 10232 in FIG. 62B, a firstinsulating film 10302 in FIG. 63A, and a first insulating film 10332 inFIG. 63B, an insulating film such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride (SiOxNy) film can be used.Alternatively, an insulating film having a stacked-layer structure inwhich two or more of a silicon oxide film, a silicon nitride film, asilicon oxynitride (SiOxNy) film, and the like are combined can be used.

As a first conductive layer 10103 in FIG. 61, a first conductive layer10203 in FIG. 62A, a first conductive layer 10233 in FIG. 62B, a firstconductive layer 10303 in FIG. 63A, and a first conductive layer 10333in FIG. 63B, Mo, Ti, Al, Nd, Cr, or the like can be used. Alternatively,a stacked-layer structure in which two or more of Mo, Ti, Al, Nd, Cr,and the like are combined can be used.

As a second insulating film 10104 in FIG. 61, a second insulating film10204 in FIG. 62A, a second insulating film 10234 in FIG. 62B, a secondinsulating film 10304 in FIG. 63A, and a second insulating film 10334 inFIG. 63B, a thermal oxide film, a silicon oxide film, a silicon nitridefilm, a silicon oxynitride film, or the like can be used. Alternatively,a stacked-layer structure in which two or more of a thermal oxide film,a silicon oxide film, a silicon nitride film, a silicon oxynitride film,and the like are combined can be used. Note that a silicon oxide film ispreferably used as a portion in contact with a semiconductor layer. Thisis because a trap level at an interface with the semiconductor layer isdecreased when a silicon oxide film is used. Note also that a siliconnitride film is preferably used as a portion in contact with Mo. This isbecause a silicon nitride film does not oxidize Mo.

As a first semiconductor layer 10105 in FIG. 61, a first semiconductorlayer 10205 in FIG. 62A, a first semiconductor layer 10235 in FIG. 62B,a first semiconductor layer 10305 in FIG. 63A, and a first semiconductorlayer 10335 in FIG. 63B, silicon, silicon germanium (SiGe), or the likecan be used.

As a second semiconductor layer 10106 in FIG. 61, a second semiconductorlayer 10206 in FIG. 62A, a second semiconductor layer 10236 in FIG. 62B,a second semiconductor layer 10306 in FIG. 63A, and a secondsemiconductor layer 10336 in FIG. 63B, silicon including phosphorus orthe like can be used, for example.

As a light-transmitting material of a second conductive layer 10107, athird conductive layer 10109, and a fourth conductive layer 10113 inFIG. 61; a second conductive layer 10207, a third conductive layer10209, and a fourth conductive layer 10213 in FIG. 62A; a secondconductive layer 10237, a third conductive layer 10239, and a fourthconductive layer 10243 in FIG. 62B; a second conductive layer 10307 anda third conductive layer 10309 in FIG. 63A; and a second conductivelayer 10337, a third conductive layer 10339, and a fourth conductivelayer 10343 in FIG. 63B, an indium tin oxide (ITO) film formed by mixingtin oxide into indium oxide, an indium tin silicon oxide (ITSO) filmformed by mixing silicon oxide into indium tin oxide (ITO), an indiumzinc oxide (IZO) film formed by mixing zinc oxide into indium oxide, azinc oxide film, a tin oxide film, or the like can be used. Note thatIZO is a light-transmitting conductive material formed by sputteringusing a target in which zinc oxide (ZnO) of 2 to 20 wt % is mixed intoITO.

As a reflective material of the second conductive layer 10107 and thethird conductive layer 10109 in FIG. 61; the second conductive layer10207 and the third conductive layer 10209 in FIG. 62A; the secondconductive layer 10237 and the third conductive layer 10239 in FIG. 62B;the second conductive layer 10307 and the third conductive layer 10309in FIG. 63A; and the second conductive layer 10337, the third conductivelayer 10339, and the fourth conductive layer 10343 in FIG. 63B, Ti, Mo,Ta, Cr, W, Al, or the like can be used. Alternatively, a two-layerstructure in which Al and Ti, Mo, Ta, Cr, or W are stacked, or athree-layer structure in which Al is interposed between metals such asTi, Mo, Ta, Cr, and W may be used.

As the third insulating film 10108 in FIG. 61, the third insulating film10208 in FIG. 62A, the third insulating film 10238 in FIG. 62B, thethird conductive layer 10239 in FIG. 62B, the third insulating film10308 in FIG. 63A, and the third insulating film 10338 and the fourthinsulating film 10349 in FIG. 63B, an inorganic material (e.g., siliconoxide, silicon nitride, or silicon oxynitride), an organic compoundmaterial having a low dielectric constant (e.g., a photosensitive ornonphotosensitive organic resin material), or the like can be used.Alternatively, a material including siloxane can be used. Note thatsiloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substitute, an organic groupcontaining at least hydrogen (such as an alkyl group or an aryl group)is used. Alternatively, a fluoro group, or a fluoro group and an organicgroup containing at least hydrogen may be used as a substituent.

As a first alignment film 10110 and a second alignment film 10112 inFIG. 61; a first alignment film 10210 and a second alignment film 10212in FIG. 62A; a first alignment film 10240 and a second alignment film10242 in FIG. 62B; a first alignment film 10310 and a second alignmentfilm 10312 in FIG. 63A; and a first alignment film 10340 and a secondalignment film 10342 in FIG. 63B, a film of a high molecular compoundsuch as polyimide can be used.

Next, the pixel structure in the case where each liquid crystal mode andthe transistor are combined is described with reference to a top planview (a layout diagram) of the pixel.

Note that as the liquid crystal mode, a TN (Twisted Nematic) mode, anIPS (In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, anMVA (Multi-domain Vertical Alignment) mode, a PVA (Patterned VerticalAlignment) mode, an ASM (Axially Symmetric aligned Microcell) mode, anOCB (Optical Compensated Birefringence) mode, an FLC (FerroelectricLiquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode,or the like can be used.

FIG. 64 is an example of a top plan view of a pixel in the case where aTN mode and a transistor are combined. By applying the pixel structureshown in FIG. 64 to a liquid crystal display device, a liquid crystaldisplay device can be formed at low cost.

The pixel shown in FIG. 64 includes a scan line 10401, a video signalline 10402, a capacitor line 10403, a transistor 10404, a pixelelectrode 10405, and a pixel capacitor 10406.

FIG. 65A is an example of a top plan view of a pixel in the case wherean MVA mode and a transistor are combined. By applying the pixelstructure shown in FIG. 65A to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle, high response speed,and high contrast can be obtained.

The pixel shown in FIG. 65A includes a scan line 10501, a video signalline 10502, a capacitor line 10503, a transistor 10504, a pixelelectrode 10505, a pixel capacitor 10506, and a projection 10507 foralignment control.

FIG. 65B is an example of a top plan view of a pixel in the case where aPVA mode and a transistor are combined. By applying the pixel structureshown in FIG. 65B to a liquid crystal display device, a liquid crystaldisplay device having a wide viewing angle, high response speed, andhigh contrast can be obtained.

The pixel shown in FIG. 65B includes a scan line 10511, a video signalline 10512, a capacitor line 10513, a transistor 10514, a pixelelectrode 10515, a pixel capacitor 10516, and an electrode notch portion10517.

FIG. 66A is an example of a top plan view of a pixel in the case wherean IPS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 66A to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle and response speedwith low dependency on gray scale in principle can be obtained.

The pixel shown in FIG. 66A includes a scan line 10601, a video signalline 10602, a common electrode 10603, a transistor 10604, and a pixelelectrode 10605.

FIG. 66B is an example of a top plan view of a pixel in the case wherean FFS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 66B to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle and response speedwith low dependency on gray scale in principle can be obtained.

The pixel shown in FIG. 66B includes a scan line 10611, a video signalline 10612, a common electrode 10613, a transistor 10614, and a pixelelectrode 10615.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 15

In this embodiment mode, a structure and an operation of a pixel in adisplay device are described.

FIGS. 67A and 67B are timing charts showing an example of digital timegray scale drive. The timing chart of FIG. 67A shows a driving methodwhen a signal writing period (an address period) to a pixel and alight-emitting period (a sustain period) are divided.

One frame period is a period for fully displaying an image for onedisplay region. One frame period includes a plurality of subframeperiods, and one subframe period includes an address period and asustain period. Address periods Ta1 to Ta4 indicate time for writingsignals to pixels in all rows, and periods Th1 to Th4 indicate time forwriting signals to pixels in one row (or one pixel). Sustain periods Ts1to Ts4 indicate time for maintaining a lighting state or a non-lightingstate in accordance with a video signal written to the pixel, and aratio of the length of the sustain periods is set to satisfyTs1:Ts2:Ts3:Ts4=2³:2²:2¹:2⁰=8:4:2:1. A gray scale is expressed dependingon which sustain period light emission is performed.

Here, the i-th pixel row is described with reference to FIG. 67B. First,in the address period Ta1, a pixel selection signal is input to a scanline in order from a first row, and in a period Tb1(i) in the addressperiod Ta1, a pixel in the i-th row is selected. Then, while the pixelin the i-th row is selected, a video signal is input to the pixel in thei-th row from a signal line. Then, when the video signal is written tothe pixel in the i-th row, the pixel in the i-th row maintains thesignal until a signal is input again. Lighting and non-lighting of thepixel in the i-th row in the sustain period Ts1 are controlled by thewritten video signal. Similarly, in the address periods Ta2, Ta3, andTa4, a video signal is input to the pixel in the i-th row, and lightingand non-lighting of the pixel in the i-th row in the sustain periodsTs2, Ts3, and Ts4 are controlled by the video signal. Then, in eachsubframe period, a pixel to which a signal for not lighting in theaddress period and for lighting when the sustain period starts after theaddress period ends is written is lit.

Here, the case where a 4-bit gray scale is expressed; however, thenumber of bits and the number of gray scales are not limited thereto.Note that lighting is not needed to be performed in order of Ts1, Ts2,Ts3, and Ts4, and the order may be random or light emission may beperformed in the period divided into a plurality of periods. A ratio oflighting time of Ts1, Ts2, Ts3, and Ts4 is not needed to bepower-of-two, and may be the same length or slightly different from apower of two.

Next, a driving method when a signal writing period (an address period)to a pixel and a light-emitting period (a sustain period) are notdivided is described. A pixel in a row in which a writing operation of avideo signal is completed maintains the signal until another signal iswritten to the pixel (or the signal is erased). Data holding time is aperiod between the writing operation and until another signal is writtento the pixel. In the data holding time, the pixel is lit or not lit inaccordance with the video signal written to the pixel. The sameoperations are performed until the last row, and the address periodends. Then, an operation proceeds to a signal writing operation in anext subframe period sequentially from a row in which the data holdingtime ends.

As described above, in the case of a driving method in which a pixel islit or not lit in accordance with a video signal written to the pixelimmediately after the signal writing operation is completed and the dataholding time starts, signals cannot be input to two rows at the sametime. Accordingly, address periods need to be prevented fromoverlapping. Therefore, the data holding time cannot be made shorterthan the address period. As a result, it becomes difficult to performhigh-level gray scale display.

Thus, the data holding time is set to be shorter than the address periodby providing an erasing period. FIG. 68A shows a driving method when thedata holding time is set shorter than the address period by providing anerasing period.

Here, the i-th pixel row is described with reference to FIG. 68B. In theaddress period Ta1, a pixel scan signal is input to a scan line in orderfrom a first row, and a pixel is selected. Then, in the period Tb1(i),while the pixel in the i-th row is selected, a video signal is input tothe pixel in the i-th row. Then, when the video signal is written to thepixel in the i-th row, the pixel in the i-th row maintains the signaluntil a signal is input again. Lighting and non-lighting of the pixel inthe i-th row in the sustain period Ts1(i) are controlled by the writtenvideo signal. That is, the pixel in the i-th row is lit or not lit inaccordance with the video signal written to the pixel immediately afterthe writing operation of the video signal to the i-th row is completed.Similarly, in the address periods Ta2, Ta3, and Ta4, a video signal isinput to the pixel in the i-th row, and lighting and non-lighting of thepixel in the i-th row in the sustain periods Ts2, Ts3, and Ts4 arecontrolled by the video signal. Then, the end of a sustain period Ts4(i)is set by the start of an erasing operation. This is because the pixelis forced to be not lit regardless of the video signal written to thepixel in the i-th row in an erasing time Te(i). That is, the dataholding time of the pixel in the i-th row ends when the erasing timeTe(i) starts.

Thus, a display device with a high-level gray scale, a high duty ratio(a ratio of a lighting period in one frame period) can be provided, inwhich data holding time is shorter than an address period withoutdividing the address period and a sustain period can be provided.Reliability of a display element can be improved since instantaneousluminance can be lowered.

Here, the case where a 4-bit gray scale is expressed; however, thenumber of bits and the number of gray scales are not limited thereto.Note that lighting is not needed to be performed in order of Ts1, Ts2,Ts3, and Ts4, and the order may be random or light emission may beperformed in the period divided into a plurality of periods. A ratio oflighting time of Ts1, Ts2, Ts3, and Ts4 is not needed to bepower-of-two, and may be the same length or slightly different from apower of two.

A structure and an operation of a pixel to which digital time gray scaledrive can be applied are described.

FIG. 69 is a diagram showing an example of a pixel structure to whichdigital time gray scale drive can be applied.

A pixel 80300 includes a switching transistor 80301, a drivingtransistor 80302, a light-emitting element 80304, and a capacitor 80303.A gate of the switching transistor 80301 is connected to a scan line80306, a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 80301 is connected to a signalline 80305, and a second electrode (the other of the source electrodeand the drain electrode) of the switching transistor 80301 is connectedto a gate of the driving transistor 80302. The gate of the drivingtransistor 80302 is connected to a power supply line 80307 through thecapacitor 80303, a first electrode of the driving transistor 80302 isconnected to the power supply line 80307, and a second electrode of thedriving transistor 80302 is connected to a first electrode (a pixelelectrode) of the light-emitting element 80304. A second electrode ofthe light-emitting element 80304 corresponds to a common electrode80308.

The second electrode of the light-emitting element 80304 (the commonelectrode 80308) is set to a low power supply potential. The low powersupply potential is a potential satisfying the low power supplypotential<a high power supply potential based on the high power supplypotential set to the power supply line 80307. As the low power supplypotential, GND, 0 V, and the like may be employed, for example. Apotential difference between the high power supply potential and the lowpower supply potential is applied to the light-emitting element 80304,and a current is supplied to the light-emitting element 80304. Here, inorder to make the light-emitting element 80304 emit light, eachpotential is set so that the potential difference between the high powersupply potential and the low power supply potential is a forwardthreshold voltage or more.

Gate capacitance of the driving transistor 80302 may be used as asubstitute for the capacitor 80303, so that the capacitor 80303 can beomitted. The gate capacitance of the driving transistor 80302 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel region and the gate electrode.

In the case of voltage-input voltage driving method, a video signal isinput to the gate of the driving transistor 80302 so that the drivingtransistor 80302 is in either of two states of being sufficiently turnedon and turned off. That is, the driving transistor 80302 operates in alinear region.

The video signal such that the driving transistor 80302 operates in asaturation region is input, so that a current can be supplied to thelight-emitting element 80304. When the light-emitting element 80304 isan element luminance of which is determined in accordance with acurrent, luminance decay due to deterioration of the light-emittingelement 80304 can be suppressed. Further, when the video signal is ananalog signal, a current corresponding to the video signal can besupplied to the light-emitting element 80304. In this case, analog grayscale drive can be performed.

A structure and an operation of a pixel called a threshold voltagecompensation pixel are described. A threshold voltage compensation pixelcan be applied to digital time gray scale drive and analog gray scaledrive.

FIG. 70 is a diagram showing an example of a structure of a pixel calleda threshold voltage compensation pixel.

The pixel in FIG. 70 includes a driving transistor 80600, a first switch80601, a second switch 80602, a third switch 80603, a first capacitor80604, a second capacitor 80605, and a light-emitting element 80620. Agate of the driving transistor 80600 is connected to a signal line 80611through the first capacitor 80604 and the first switch 80601 in thisorder. Further, the gate of the driving transistor 80600 is connected toa power supply line 80612 through the second capacitor 80605. A firstelectrode of the driving transistor 80600 is connected to the powersupply line 80612. A second electrode of the driving transistor 80600 isconnected to a first electrode of the light-emitting element 80620through the third switch 80603. Further, the second electrode of thedriving transistor 80600 is connected to the gate of the drivingtransistor 80600 through the first electrode of the light-emittingelement 80620. A second electrode of the light-emitting element 80620corresponds to a common electrode 80621. Note that on/off of the firstswitch 80601, the second switch 80602, and the third switch 80603 iscontrolled by a signal input to a first scan line 80613, a signal inputto a second scan line 80615, and a signal input to a third scan line80614, respectively.

A pixel structure shown in FIG. 70 is not limited thereto. For example,a switch, a resistor, a capacitor, a transistor, a logic circuit, or thelike may be added to the pixel in FIG. 70. For example, the secondswitch 80602 may include a p-channel transistor or an n-channeltransistor, the third switch 80603 may include a transistor withpolarity opposite to that of the second switch 80602, and the secondswitch 80602 and the third switch 80603 may be controlled by the samescan line.

A structure and an operation of a pixel called a current input pixel aredescribed. A current input pixel can be applied to digital gray scaledrive and analog gray scale drive. FIG. 71 is a diagram showing anexample of a structure of a pixel called a current input pixel.

The pixel in FIG. 71 includes a driving transistor 80700, a first switch80701, a second switch 80702, a third switch 80703, a capacitor 80704,and a light-emitting element 80730. A gate of the driving transistor80700 is connected to a signal line 80711 through the second switch80702 and the first switch 80701 in this order. Further, the gate of thedriving transistor 80700 is connected to a power supply line 80712through the capacitor 80704. A first electrode of the driving transistor80700 is connected to the power supply line 80712. A second electrode ofthe driving transistor 80700 is connected to the signal line 80711through the first switch 80701. Further, the second electrode of thedriving transistor 80700 is connected to a first electrode of thelight-emitting element 80730 through the third switch 80703. A secondelectrode of the light-emitting element 80730 corresponds to a commonelectrode 80731. Note that on/off of the first switch 80701, the secondswitch 80702, and the third switch 80703 is controlled by a signal inputto a first scan line 80713, a signal input to a second scan line 80714,and a signal input to a third scan line 80715, respectively.

A pixel structure shown in FIG. 71 is not limited thereto. For example,a switch, a resistor, a capacitor, a transistor, a logic circuit, or thelike may be added to the pixel in FIG. 71. For example, the first switch80701 may include a p-channel transistor or an n-channel transistor, thesecond switch 80702 may include a transistor with the same polarity asthat of the first switch 80701, and the first switch 80701 and thesecond switch 80702 may be controlled by the same scan line. The secondswitch 80702 may be provided between the gate of the driving transistor80700 and the signal line 80711.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 16

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a display device using anorganic EL element is described.

FIG. 72A shows an example of a top plan view (a layout diagram) of apixel including two transistors. FIG. 72B shows an example of across-sectional view along X-X′ in FIG. 72A.

FIGS. 72A and 72B show a first transistor 60105, a first wiring 60106, asecond wiring 60107, a second transistor 60108, a third wiring 60111, anopposite electrode 60112, a capacitor 60113, a pixel electrode 60115, apartition wall 60116, an organic conductive film 60117, an organic thinfilm 60118, and a substrate 60119. Note that it is preferable that thefirst transistor 60105 be used as a switching transistor, the secondtransistor 60108 as a driving transistor, the first wiring 60106 as agate signal line, the second wiring 60107 as a source signal line, andthe third wiring 60111 as a current supply line.

A gate electrode of the first transistor 60105 is electrically connectedto the first wiring 60106, one of a source electrode and a drainelectrode of the first transistor 60105 is electrically connected to thesecond wiring 60107, and the other of the source electrode or the drainelectrode of the first transistor 60105 is electrically connected to agate electrode of the second transistor 60108 and one electrode of thecapacitor 60113. Note that the gate electrode of the first transistor60105 includes a plurality of gate electrodes. Accordingly, a leakagecurrent in the off state of the first transistor 60105 can be reduced.

One of a source electrode and a drain electrode of the second transistor60108 is electrically connected to the third wiring 60111, and the otherof the source electrode or the drain electrode of the second transistor60108 is electrically connected to the pixel electrode 60115.Accordingly, a current flowing to the pixel electrode 60115 can becontrolled by the second transistor 60108.

The organic conductive film 60117 is provided over the pixel electrode60115, and the organic thin film 60118 (an organic compound layer) isfurther provided thereover. The opposite electrode 60112 is providedover the organic thin film 60118 (the organic compound layer). Note thatthe opposite electrode 60112 may be formed over a surface of all pixelsto be commonly connected to all the pixels, or may be patterned using ashadow mask or the like.

Light emitted from the organic thin film 60118 (the organic compoundlayer) is transmitted through either the pixel electrode 60115 or theopposite electrode 60112.

In FIG. 72B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe opposite electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60115 be formed of a light-transmitting conductive film. Inthe case of top emission, it is preferable that the opposite electrode60112 be formed of a light-transmitting conductive film.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surfaceuniformly and light emission of RGB can be obtained by using a colorfilter.

Note that the structures shown in FIGS. 72A and 72B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, as well as the structures shown in FIGS. 72A and 72B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawing.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 17

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an organic EL element is described.

A structure of a mixed junction EL element is described. As an example,a structure is described, which includes a layer (a mixed layer) inwhich a plurality of materials among a hole injecting material, a holetransporting material, a light-emitting material, an electrontransporting material, an electron injecting material, and the like aremixed (hereinafter referred to as a mixed junction type EL element),which is different from a stacked-layer structure where a hole injectinglayer formed of a hole injecting material, a hole transporting layerformed of a hole transporting material, a light-emitting layer formed ofa light-emitting material, an electron transporting layer formed of anelectron transporting material, an electron injecting layer formed of anelectron injecting material, and the like are clearly distinguished.

FIGS. 73A to 73E are schematic views each showing a structure of a mixedjunction type EL element. Note that a layer interposed between the anode190101 and the cathode 190102 corresponds to an EL layer.

In the structure shown in FIG. 73A, the EL layer includes the EL layerincludes a hole transporting region 190103 formed of a hole transportingmaterial and an electron transporting region 190104 formed of anelectron transporting material. The hole transporting region 190103 iscloser to the anode than the electron transporting region 190104. Amixed region 190105 including both the hole transporting material andthe electron transporting material is provided between the holetransporting region 190103 and the electron transporting region 190104.

In the direction from the anode 190101 to the cathode 190102, aconcentration of the hole transporting material in the mixed region190105 is decreased and a concentration of the electron transportingmaterial in the mixed region 190105 is increased.

A concentration gradient can be freely set. For example, a ratio ofconcentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material. Alternatively, aratio of concentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material and the electrontransporting layer 190104 formed of only the electron transportingmaterial. A ratio of concentrations may be changed depending on adistance from the anode or the cathode. Further, the ratio ofconcentrations may be changed continuously.

A region 190106 to which a light-emitting material is added is includedin the mixed region 190105. A light emission color of the EL element canbe controlled by the light-emitting material. Further, carriers can betrapped by the light-emitting material. As the light-emitting material,various fluorescent dyes as well as a metal complex having a quinolineskeleton, a benzooxazole skeleton, or a benzothiazole skeleton can beused. The light emission color of the EL element can be controlled byadding the light-emitting material.

As the anode 190101, an electrode material having a high work functionis preferably used in order to inject holes efficiently. For example, atransparent electrode formed of indium tin oxide (ITO), indium zincoxide (IZO), ZnO, SnO₂, In₂O₃, or the like can be used. When alight-transmitting property is not needed, the anode 190101 may beformed of an opaque metal material.

As the hole transporting material, an aromatic amine compound or thelike can be used.

As the electron transporting material, a metal complex having aquinoline derivative, 8-quinolinol, or a derivative thereof as a ligand(especially tris(8-quinolinolato)aluminum (Alq₃)), or the like can beused.

As the cathode 190102, an electrode material having a low work functionis preferably used in order to inject electrons efficiently. Forexample, a metal such as aluminum, indium, magnesium, silver, calcium,barium, or lithium can be used by itself. Alternatively, an alloy of theaforementioned metal or an alloy of the aforementioned metal and anothermetal may be used.

FIG. 73B is the schematic view of the structure of the EL element, whichis different from that of FIG. 73A. Note that the same portions as thosein FIG. 73A are denoted by the same reference numerals, and descriptionthereof is omitted.

In FIG. 73B, a region to which a light-emitting material is added is notincluded. However, when a material (electron-transporting andlight-emitting material) having both an electron transporting propertyand a light-emitting property, for example,tris(8-quinolinolato)aluminum (Alq₃) is used as a material added to theelectron transporting region 190104, light emission can be performed.

Alternatively, as a material added to the hole transporting region190103, a material (a hole-transporting and light-emitting material)having both a hole transporting property and a light-emitting propertymay be used.

FIG. 73C is the schematic view of the structure of the EL element, whichis different from those of FIGS. 73A and 73B. Note that the sameportions as those in FIGS. 73A and 73B are denoted by the same referencenumerals, and description thereof is omitted.

In FIG. 73C, a region 190107 included in the mixed region 190105 isprovided, to which a hole blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the hole transporting material isadded. The region 190107 to which the hole blocking material is added isprovided closer to the cathode 190102 than the region 190106 to whichthe light-emitting material is added in the mixed region 190105; thus, arecombination rate of carriers and light emission efficiency can beincreased. The aforementioned structure provided with the region 190107to which the hole blocking material is added is especially effective inan EL element which utilizes light emission (phosphorescence) by atriplet exciton.

FIG. 73D is the schematic view of the structure of the EL element, whichis different from those of FIGS. 73A to 73C. Note that the same portionsas those in FIGS. 73A to 73C are denoted by the same reference numerals,and description thereof is omitted.

In FIG. 73D, a region 190108 included in the mixed region 190105 isprovided, to which an electron blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the electron transporting material isadded. The region 190108 to which the electron blocking material isadded is provided closer to the anode 190101 than the region 190106 towhich the light-emitting material is added in the mixed region 190105;thus, a recombination rate of carriers and light emission efficiency canbe increased. The aforementioned structure provided with the region190108 to which the electron blocking material is added is especiallyeffective in an EL element which utilizes light emission(phosphorescence) by a triplet exciton.

FIG. 73E is the schematic view of the structure of the mixed junctiontype EL element, which is different from those of FIGS. 73A to 73D. FIG.73E shows an example of a structure where a region 190109 to which ametal material is added is included in part of an EL layer in contactwith an electrode of the EL element. In FIG. 73E, the same portions asthose in FIGS. 73A to 73D are denoted by the same reference numerals,and description thereof is omitted. In FIG. 73E, MgAg (an Mg—Ag alloy)may be used as the cathode 190102, and the region 190109 to which Al(aluminum) alloy is added may be included in a region of the electrontransporting region 190104 to which the electron transporting materialis added, which is in contact with the cathode 190102, for example. Bythe aforementioned structure, oxidation of the cathode can be prevented,and electron injection efficiency from the cathode can be increased.Therefore, the lifetime of the mixed junction type EL element can beextended, and a driving voltage can be lowered.

As a method of forming the aforementioned mixed junction type ELelement, a co-evaporation method or the like can be used.

In the mixed junction type EL elements as shown in FIGS. 73A to 73E, aclear interface between the layers does not exist, and chargeaccumulation can be reduced. Thus, the lifetime of the EL element can beextended, and a driving voltage can be lowered.

Note that the structures shown in FIGS. 73A to 73E can be implemented infree combination with each other.

A structure of the mixed junction type EL element is not limited tothose described above, and various structures can be freely used.

An organic material which forms an EL layer of an EL element may be alow molecular material or a high molecular material, and both of thematerials may be used. When a low molecular material is used as anorganic compound material, a film can be formed by an evaporationmethod. When a high molecular material is used as the EL layer, the highmolecular material is dissolved in a solvent and a film can be formed bya spin coating method or an ink-jet method.

The EL layer may be formed of a middle molecular material. In thisspecification, a middle molecule organic light-emitting material denotesan organic light-emitting material without a sublimation property andwith a polymerization degree of approximately 20 or less. When a middlemolecular material is used as the EL layer, a film can be formed by anink-jet method or the like.

A low molecular material, a high molecular material, and a middlemolecular material may be used in combination.

An EL element may utilize either light emission (fluorescence) by asinglet exciton or light emission (phosphorescence) by a tripletexciton.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 18

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an inorganic EL element is described.

As a base material to be used for a light-emitting material, sulfide,oxide, or nitride can be used. As sulfide, zinc sulfide (ZnS), cadmiumsulfide (CdS), calcium sulfide (CaS), yttrium sulfide (Y₂S₃), galliumsulfide (Ga₂S₃), strontium sulfide (SrS), barium sulfide (BaS), or thelike can be used, for example. As oxide, zinc oxide (ZnO), yttrium oxide(Y₂O₃), or the like can be used, for example. As nitride, aluminumnitride (AlN), gallium nitride (GaN), indium nitride (InN), or the likecan be used, for example. Further, zinc selenide (ZnSe), zinc telluride(ZnTe), or the like; or a ternary mixed crystal such as calcium galliumsulfide (CaGa₂S₄), strontium gallium sulfide (SrGa₂S₄), or bariumgallium sulfide (BaGa₂S₄) may be used.

As a luminescence center for localized light emission, manganese (Mn),copper (Cu), samarium (Sm), terbium (Tb), erbium (Er), thulium (Tm),europium (Eu), cerium (Ce), praseodymium (Pr), or the like can be used.Further, a halogen element such as fluorine (F) or chlorine (Cl) may beadded for charge compensation.

On the other hand, as a luminescence center for donor-acceptorrecombination light emission, a light-emitting material including afirst impurity element forming a donor level and a second impurityelement forming an acceptor level can be used. As the first impurityelement, fluorine (F), chlorine (Cl), aluminum (Al), or the like can beused, for example. As the second impurity element, copper (Cu), silver(Ag), or the like can be used, for example. FIGS. 74A to 74C each showan example of a thin-film type inorganic EL element which can be used asa light-emitting element. In FIGS. 74A to 74C, the light-emittingelement includes a first electrode layer 120100, an electroluminescentlayer 120102, and a second electrode layer 120103.

The light-emitting elements in FIGS. 74B and 74C each have a structurewhere an insulating film is provided between the electrode layer and theelectroluminescent layer in the light-emitting element in FIG. 74A. Thelight-emitting element in FIG. 74B includes an insulating film 120104between the first electrode layer 120100 and the electroluminescentlayer 120102. The light-emitting element in FIG. 74C includes aninsulating film 120105 between the first electrode layer 120100 and theelectroluminescent layer 120102, and an insulating film 120106 betweenthe second electrode layer 120103 and the electroluminescent layer120102. Accordingly, the insulating film may be provided between theelectroluminescent layer and one of the electrode layers interposing theelectroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers interposingthe electroluminescent layer. Further, the insulating film may be asingle layer or stacked layers including a plurality of layers.

FIGS. 75A to 75C each show an example of a dispersion type inorganic ELelement which can be used as a light-emitting element. A light-emittingelement in FIG. 75A has a stacked-layer structure of a first electrodelayer 120200, an electroluminescent layer 120202, and a second electrodelayer 120203. The electroluminescent layer 120202 includes alight-emitting material 120201 held by a binder.

The light-emitting elements in FIGS. 75B and 75C each have a structurewhere an insulating film is provided between the electrode layer and theelectroluminescent layer in the light-emitting element in FIG. 75A. Thelight-emitting element in FIG. 75B includes an insulating film 120204between the first electrode layer 120200 and the electroluminescentlayer 120202. The light-emitting element in FIG. 75C includes aninsulating film 120205 between the first electrode layer 120200 and theelectroluminescent layer 120202, and an insulating film 120206 betweenthe second electrode layer 120203 and the electroluminescent layer120202. Accordingly, the insulating film may be provided between theelectroluminescent layer and one of the electrode layers interposing theelectroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers interposingthe electroluminescent layer. Further, the insulating film may be asingle layer or stacked layers including a plurality of layers.

The insulating film 120204 is provided in contact with the firstelectrode layer 120200 in FIG. 75B; however, the insulating film 120204may be provided in contact with the second electrode layer 120203 byreversing the positions of the insulating film and theelectroluminescent layer.

It is preferable that a material which can be used for the insulatingfilms such as the insulating film 120104 in FIG. 74B and the insulatingfilm 120204 in FIG. 75B has high withstand voltage and dense filmquality. Further, the material preferably has high dielectric constant.For example, silicon oxide (SiO₂), yttrium oxide (Y₂O₃), titanium oxide(TiO₂), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), tantalum oxide(Ta₂O₅), barium titanate (BaTiO₃), strontium titanate (SrTiO₃), leadtitanate (PbTiO₃), silicon nitride (Si₃N₄), or zirconium oxide (ZrO₂);or a mixed film of those materials or a stacked-layer film including twoor more of those materials can be used. The insulating film can beformed by sputtering, evaporation, CVD, or the like. Alternatively, theinsulating film may be formed by dispersing particles of theseinsulating materials in a binder. A binder material may be formed usinga material similar to that of a binder contained in theelectroluminescent layer, by using a method similar thereto. Thethickness of the insulating film is not particularly limited, butpreferably in the range of 10 to 1000 nm.

The light-emitting element can emit light when a voltage is appliedbetween the pair of electrode layers interposing the electroluminescentlayer. The light-emitting element can operate with DC drive or AC drive.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to; combinedwith, or replaced with this embodiment mode.

Embodiment Mode 19

In this embodiment mode, an example of a display device is described. Inparticular, the case where a display device is optically treated isdescribed.

A rear projection display device 130100 in FIGS. 76A and 76B is providedwith a projector unit 130111, a mirror 130112, and a screen panel130101. The rear projection display device 130100 may also be providedwith a speaker 130102 and operation switches 130104. The projector unit130111 is provided at a lower portion of a housing 130110 of the rearprojection display device 130100, and projects incident light forprojecting an image based on a video signal to the mirror 130112. Therear projection display device 130100 displays an image projected from arear surface of the screen panel 130101.

FIG. 77 shows a front projection display device 130200. The frontprojection display device 130200 is provided with the projector unit130111 and a projection optical system 130201. The projection opticalsystem 130201 projects an image to a screen or the like provided at thefront.

Hereinafter, a structure of the projector unit 130111 which is appliedto the rear projection display device 130100 in FIGS. 76A and 76B andthe front projection display device 130200 in FIG. 77 is described.

FIG. 78 shows a structure example of the projector unit 130111. Theprojector unit 130111 is provided with a light source unit 130301 and amodulation unit 130304. The light source unit 130301 is provided with alight source optical system 130303 including lenses and a light sourcelamp 130302. The light source lamp 130302 is stored in a housing so thatstray light is not scattered. As the light source lamp 130302, ahigh-pressure mercury lamp or a xenon lamp, for example, which can emita large amount of light is used. The light source optical system 130303is provided with an optical lens, a film having a function to polarizelight, a film for adjusting phase difference, an IR film, or the like asappropriate. The light source unit 130301 is provided so that incidentlight is incident on the modulation unit 130304. The modulation unit130304 is provided with a plurality of display panels 130308, a colorfilter, a dichroic mirror 130305, a total reflection mirror 130306, aretardation plate 130307, a prism 130309, and a projection opticalsystem 130310. Light emitted from the light source unit 130301 is splitinto a plurality of optical paths by the dichroic mirror 130305.

Each optical path is provided with a color filter which transmits lightwith a predetermined wavelength or wavelength range and the displaypanel 130308. The transmissive display panel 130308 modulatestransmitted light based on a video signal. Light of each colortransmitted through the display panel 130308 is incident on the prism130309, and an image is displayed on the screen through the projectionoptical system 130310. Note that a Fresnel lens may be provided betweenthe mirror and the screen. Projected light which is projected by theprojector unit 130111 and reflected by the mirror is converted intogenerally parallel light by the Fresnel lens to be projected on thescreen. Displacement between a chief ray and an optical axis ispreferably ±10° or less, and more preferably, ±5° or less.

The projector unit 130111 shown in FIG. 79 is provided with reflectivedisplay panels 130407, 130408, and 130409.

The projector unit 130111 in FIG. 79 is provided with the light sourceunit 130301 and a modulation unit 130400. The light source unit 130301may have a structure similar to FIG. 78. Light from the light sourceunit 130301 is split into a plurality of optical paths by dichroicmirrors 130401 and 130402 and a total reflection mirror 130403 to beincident on polarization beam splitters 130404, 130405, and 130406. Thepolarization beam splitters 130404, 130405, and 130406 are providedcorresponding to the reflective display panels 130407, 130408, and130409 which correspond to respective colors. The reflective displaypanels 130407, 130408, and 130409 modulate reflected light based on avideo signal. Light of each color, which are reflected by the reflectivedisplay panels 130407, 130408, and 130409, is incident on a prism 130410to be composed, and projected through a projection optical system130411.

Among light emitted from the light source unit 130301, only light in awavelength region of red is transmitted through the dichroic mirror130401 and light in wavelength regions of green and blue is reflected bythe dichroic mirror 130401. Further, only the light in the wavelengthregion of green is reflected by the dichroic mirror 130402. The light inthe wavelength region of red, which is transmitted through the dichroicmirror 130401, is reflected by the total reflection mirror 130403 andincident on the polarization beam splitter 130404. The light in thewavelength region of blue is incident on the polarization beam splitter130405. The light in the wavelength region of green is incident on thepolarization beam splitter 130406. The polarization beam splitters130404, 130405, and 130406 have a function to split incident light intoP-polarized light and S-polarized light and a function to transmit onlyP-polarized light. The reflective display panels 130407, 130408, and130409 polarize incident light based on a video signal.

Only the S-polarized light corresponding to each color is incident onthe reflective display panels 130407, 130408, and 130409 correspondingto each color. Note that the reflective display panels 130407, 130408,and 130409 may be liquid crystal panels. In this case, the liquidcrystal panel operates in an electrically controlled birefringence (ECB)mode. Liquid crystal molecules are vertically aligned at an angle to asubstrate. Accordingly, in the reflective display panels 130407, 130408,and 130409, when a pixel is turned off, display molecules are alignednot to change a polarization state of incident light so as to reflectthe incident light. When the pixel is turned on, alignment of thedisplay molecules is changed, and the polarization state of the incidentlight is changed.

The projector unit 130111 in FIG. 79 can be applied to the rearprojection display device 130100 in FIGS. 76A and 76B and the frontprojection display device 130200 in FIG. 77.

FIGS. 80A to 80C each show a single-panel type projector unit. Theprojector unit 130111 shown in FIG. 80A is provided with the lightsource unit 130301, a display panel 130507, a projection optical system130511, and a retardation plate 130504. The projection optical system130511 includes one or a plurality of lenses. The display panel 130507may be provided with a color filter. FIG. 80B shows a structure of theprojector unit 130111 operating in a field sequential mode. The fieldsequential mode corresponds to a mode in which color display isperformed by light of respective colors such as red, green, and bluesequentially incident on a display panel with a time lag, without acolor filter. A high definition image can be displayed particularly bycombination with a display panel with high-speed response to change ininput signal. The projector unit 130111 in FIG. 80B is provided with arotating color filter plate 130505 including a plurality of colorfilters with red, green, blue, or the like between the light source unit130301 and a display panel 130508. FIG. 80C shows a structure of theprojector unit 130111 with a color separation system using a micro lens,as a color display method. The color separation system corresponds to asystem in which color display is realized by providing a micro lensarray 130506 on the side of a display panel 130509, on which light isincident, and light of each color is emitted from each direction.

The projector unit 130111 employing this system has little loss of lightdue to a color filter, so that light from the light source unit 130301can be efficiently utilized. The projector unit 130111 in FIG. 80C isprovided with dichroic mirrors 130501, 130502, and 130503 so that lightof each color is emitted to the display panel 130509 from eachdirection.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 20

In this embodiment mode, examples of electronic devices are described.

FIG. 81 shows a display panel module combining a display panel 900101and a circuit board 900111. The display panel 900101 includes a pixelportion 900102, a scan line driver circuit 900103, and a signal linedriver circuit 900104. The circuit board 900111 is provided with acontrol circuit 900112, a signal dividing circuit 900113, and the like,for example. The display panel 900101 and the circuit board 900111 areconnected to each other by a connection wiring 900114. An FPC or thelike can be used as the connection wiring.

FIG. 86 is a block diagram showing a main structure of a televisionreceiver. A tuner 900201 receives a video signal and an audio signal.The video signals are processed by an video signal amplifier circuit900202; a video signal processing circuit 900203 which converts a signaloutput from the video signal amplifier circuit 900202 into a colorsignal corresponding to each color of red, green and blue; and a controlcircuit 900212 which converts the video signal into the inputspecification of a driver circuit. The control circuit 900212 outputs asignal to each of a scan line driver circuit 900214 and a signal linedriver circuit 900204. The scan line driver circuit 900214 and thesignal line driver circuit 900204 drive a display panel 900211. Whenperforming digital drive, a structure may be employed in which a signaldividing circuit 900213 is provided on the signal line side so that aninput digital signal is divided into m signals (m is a positive integer)to be supplied.

Among the signals received by the tuner 900201, an audio signal istransmitted to an audio signal amplifier circuit 900205, and an outputthereof is supplied to a speaker 900207 through an audio signalprocessing circuit 900206. A control circuit 900208 receives controlinformation on receiving station (receiving frequency) and volume froman input portion 900209 and transmits signals to the tuner 900201 or theaudio signal processing circuit 900206.

FIG. 87A shows a television receiver incorporated with a display panelmodule, which is different from FIG. 86. In FIG. 87A, a display screen900302 incorporated in a housing 900301 is formed using the displaypanel module. Note that speakers 900303, input means (an operation key900304, a connection terminal 900305, a sensor 900306 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 900307), and the like may be provided as appropriate.

FIG. 87B shows a television receiver in which only a display can becarried wirelessly. The television receiver is provided with a displayportion 900313, a speaker portion 900317, input means (an operation key900316, a connection terminal 900318, a sensor 900319 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 900320), and the like as appropriate. A battery and a signalreceiver are incorporated in a housing 900312. The battery drives thedisplay portion 900313, the speaker portion 900317, the sensor 900319,and the microphone 900320. The battery can be repeatedly charged by acharger 900310. The charger 900310 can transmit and receive a videosignal and transmit the video signal to the signal receiver of thedisplay. The device in FIG. 87B is controlled by the operation key900316. Alternatively, the device in FIG. 87B can transmit a signal tothe charger 900310 by operating the operation key 900316. That is, thedevice may be an image and audio interactive communication device.Further alternatively, by operating the operation key 900316, the devicein FIG. 87B may transmit a signal to the charger 900310 and anotherelectronic device is made to receive a signal which can be transmittedfrom the charger 900310; thus, the device in FIG. 87B can controlcommunication of another electronic device. That is, the device may be ageneral-purpose remote control device. Note that the contents (or partthereof) described in each drawing of this embodiment mode can beapplied to the display portion 900313.

Next, a structure example of a mobile phone is described with referenceto FIG. 88.

A display panel 900501 is detachably incorporated in a housing 900530.The shape and size of the housing 900530 can be changed as appropriatein accordance with the size of the display panel 900501. The housing900530 which fixes the display panel 900501 is fitted in a printedwiring board 900531 to be assembled as a module.

The display panel 900501 is connected to the printed wiring board 900531through an FPC 900513. The printed wiring board 900531 is provided witha speaker 900532, a microphone 900533, a transmitting/receiving circuit900534, a signal processing circuit 900535 including a CPU, acontroller, and the like, and a sensor 900541 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray). Such a module,an operation key 900536, a battery 900537, and an antenna 900540 arecombined and stored in a housing 900539. A pixel portion of the displaypanel 900501 is provided to be seen from an opening window formed in thehousing 900539.

In the display panel 900501, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over the same substrate byusing transistors, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 900501 by COG (Chip On Glass).Alternatively, the IC chip may be connected to a glass substrate byusing TAB (Tape Automated Bonding) or a printed wiring board. With sucha structure, power consumption of a display device can be reduced andoperation time of the mobile phone per charge can be extended. Further,reduction in cost of the mobile phone can be realized.

The mobile phone in FIG. 88 has various functions such as, but notlimited to, a function to display various kinds of information (e.g., astill image, a moving image, and a text image); a function to display acalendar, a date, the time, and the like on a display portion; afunction to operate or edit the information displaying on the displayportion; a function to control processing by various kinds of software(programs); a function of wireless communication; a function tocommunicate with another mobile phone, a fixed phone, or an audiocommunication device by using the wireless communication function; afunction to connect with various computer networks by using the wirelesscommunication function; a function to transmit or receive various kindsof data by using the wireless communication function; a function tooperate a vibrator in accordance with incoming call, reception of data,or an alarm; and a function to generate a sound in accordance withincoming call, reception of data, or an alarm.

FIG. 89A shows a display, which includes a housing 900711, a supportbase 900712, a display portion 900713, a speaker 900717, an LED lamp900719, input means (a connection terminal 900714, a sensor 900715(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), a microphone 900716, and an operation key 900718), andthe like. The display in FIG. 89A can have various functions such as,but not limited to, a function to display various kinds of information(e.g., a still image, a moving image, and a text image) on the displayportion.

FIG. 89B shows a camera, which includes a main body 900731, a displayportion 900732, a shutter button 900736, a speaker 900740, an LED lamp900741, input means (an image receiving portion 900733, operation keys900734, an external connection port 900735, a connection terminal900737, a sensor 900738 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 900739), and thelike. The camera in FIG. 89B can have various functions such as, but notlimited to, a function to photograph a still image or a moving image; afunction to automatically adjust the photographed image (the still imageor the moving image); a function to store the photographed image in arecording medium (provided externally or incorporated in the camera);and a function to display the photographed image on the display portion.

FIG. 89C shows a computer, which includes a main body 900751, a housing900752, a display portion 900753, a speaker 900760, an LED lamp 900761,a reader/writer 900762, input means (a keyboard 900754, an externalconnection port 900755, a pointing device 900756, a connection terminal900757, a sensor 900758 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 900759), and thelike. The computer in FIG. 89C can have various functions such as, butnot limited to, a function to display various kinds of information(e.g., a still image, a moving image, and a text image) on the displayportion; a function to control processing by various kinds of software(programs); a communication function such as wireless communication orwire communication; a function to connect with various computer networksby using the communication function; and a function to transmit orreceive various kinds of data by using the communication function.

FIG. 96A shows a mobile computer, which includes a main body 901411, adisplay portion 901412, a switch 901413, a speaker 901419, an LED lamp901420, input means (operation keys 901414, an infrared port 901415, aconnection terminal 901416, a sensor 901417 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901418), and the like. The mobile computer in FIG. 96A canhave various functions such as, but not limited to, a function todisplay various kinds of information (e.g., a still image, a movingimage, and a text image) on a display portion; a touch panel functionprovided on the display portion; a function to display a calendar, adate, the time, and the like on the display portion; a function tocontrol processing by various kinds of software (programs); a functionof wireless communication; a function to connect with various computernetworks by using the wireless communication function; and a function totransmit or receive various kinds of data by using the wirelesscommunication function.

FIG. 96B shows a portable image reproducing device having a recordingmedium (e.g., a DVD reproducing device), which includes a main body901431, a housing 901432, a display portion A 901433, a display portionB 901434, a speaker portion 901437, an LED lamp 901441, input means (arecording medium (e.g., DVD) reading portion 901435, operation keys901436, a connection terminal 901438, a sensor 901439 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901440), and the like. The display portion A 901433 mainlydisplays image information and the display portion B 901434 mainlydisplays text information.

FIG. 96C shows a goggle-type display, which includes a main body 901451,a display portion 901452, an earphone 901453, a support portion 901454,an LED lamp 901459, a speaker 901458, input means (a connection terminal901455, a sensor 901456 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 901457), and thelike. The goggle-type display in FIG. 96C can have various functionssuch as, but not limited to, a function to display an externallyobtained image (e.g., a still image, a moving image, and a text image)on the display portion.

FIG. 97A shows a portable game machine, which includes a housing 901511,a display portion 901512, a speaker portion 901513, a recording mediuminsert portion 901515, an LED lamp 901519, input means (an operation key901514, a connection terminal 901516, a sensor 901517 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901518), and the like. The portable game machine in FIG. 97Acan have various functions such as, but not limited to, a function toread a program or data stored in the recording medium to display on thedisplay portion; and a function to share information by wirelesscommunication with another portable game machine.

FIG. 97B shows a digital camera having a television reception function,which includes a housing 901531, a display portion 901532, a speaker901534, a shutter button 901535, an LED lamp 901541, input means (anoperation key 901533, an image receiving portion 901536, an antenna901537, a connection terminal 901538, a sensor 901539 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901540), and the like. The digital camera having a televisionreception function in FIG. 97B can have various functions such as, butnot limited to, a function to photograph a still image or a movingimage; a function to automatically adjust the photographed image; afunction to obtain various kinds of information from the antenna; afunction to store the photographed image or the information obtainedfrom the antenna; and a function to display the photographed image orthe information obtained from the antenna on the display portion.

FIG. 98 shows a portable game machine, which includes a housing 901611,a first display portion 901612, a second display portion 901613, aspeaker portion 901614, a recording medium insert portion 901616, an LEDlamp 901620, input means (an operation key 901615, a connection terminal901617, a sensor 901418 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 901619), and thelike. The portable game machine in FIG. 98 can have various functionssuch as, but not limited to, a function to read a program or data storedin the recording medium to display on the display portion; and afunction to share information by wireless communication with anotherportable game machine.

As shown in FIGS. 89A to 89C, 96A to 96C, 97A, 97B, and 98, theelectronic device includes a display portion for displaying some kind ofinformation.

Next, application examples of a semiconductor device are described.

FIG. 90 shows an example where a semiconductor device is incorporated ina constructed object. FIG. 90 shows a housing 900810, a display portion900811, a remote control device 900812 which is an operation portion, aspeaker portion 900813, and the like. The semiconductor device isincorporated in the constructed object as a wall-hanging type and can beprovided without requiring a large space.

FIG. 91 shows another example where a semiconductor device isincorporated in a constructed object. A display panel 900901 isincorporated with a prefabricated bath 900902, and a person who takes abath can view the display panel 900901. The display panel 900901 has afunction to display information by an operation by a person who takes abath; and a function to be used as an advertisement or an entertainmentmeans.

The semiconductor device can be provided not only to a side wall of theprefabricated bath 900902 as shown in FIG. 91, but also to variousplaces. For example, the semiconductor device can be incorporated withpart of a mirror, a bathtub itself, or the like. At this time, a shapeof the display panel 900901 may be changed in accordance with a shape ofthe mirror or the bathtub.

FIG. 92 shows another example where a semiconductor device isincorporated in a constructed object. A display panel 901002 is bent andattached to a curved surface of a column-shaped object 901001. Here, autility pole is described as the column-shaped object 901001.

The display panel 901002 in FIG. 92 is provided at a position higherthan a human viewpoint. When the same images are displayed on thedisplay panels 901002 provided in constructed objects which standtogether in large numbers outdoors, such as utility poles, advertisementcan be performed to unspecified number of viewers. Since it is easy forthe display panel 901002 to display the same images and instantly switchimages by external control, highly effective information display andadvertisement effect can be expected. When provided with self-luminousdisplay elements, the display panel 901002 can be effectively used as ahighly visible display medium even at night. When the display panel901002 is provided in the utility pole, a power supply means for thedisplay panel 901002 can be easily obtained. In an emergency such asdisaster, the display panel 901002 can also be used as a means totransmit correct information to victims rapidly.

As the display panel 901002, a display panel in which a switchingelement such as an organic transistor is provided over a film-shapedsubstrate, and a display element is driven, so that an image can bedisplayed can be used, for example.

In this embodiment mode, a wall, a column-shaped object, and aprefabricated bath are shown as examples of a constructed object;however, this embodiment mode is not limited thereto, and variousconstructed objects can be provided with a semiconductor device.

Next, examples where a semiconductor device is incorporated with amoving object are described.

FIG. 93 shows an example where a semiconductor device is incorporatedwith a car. A display panel 901102 is incorporated with a car body901101, and can display an operation of the car body or informationinput from inside or outside the car body on demand. Note that anavigation function may be provided.

The semiconductor device can be provided not only to the car body 901101as shown in FIG. 93, but also to various places. For example, thesemiconductor device can be incorporated with a glass window, a door, asteering wheel, a gear shift, a seat, a rear-view mirror, and the like.At this time, a shape of the display panel 901102 may be changed inaccordance with a shape of an object provided with the semiconductordevice.

FIGS. 94A and 94B show examples where a semiconductor device isincorporated with a train car are described.

FIG. 94A shows an example where a display panel 901202 is provided inglass of a door 901201 in a train car, which has an advantage comparedwith a conventional advertisement using paper in that labor cost forchanging an advertisement is not necessary. Since the display panel901202 can instantly switch images displaying on a display portion by anexternal signal, images on the display panel can be switched in everytime period when types of passengers on the train are changed, forexample; thus, more effective advertisement effect can be expected.

FIG. 94B shows an example where the display panels 901202 are providedto a glass window 901203 and a ceiling 901204 as well as the glass ofthe door 901201 in the train car. In this manner, the semiconductordevice can be easily provided to a place where the semiconductor devicehas been difficult to be provided conventionally; thus, effectiveadvertisement effect can be obtained. Further, the semiconductor devicecan instantly switch images displayed on a display portion by anexternal signal; thus, cost and time for changing an advertisement canbe reduced, and more flexible advertisement management and informationtransmission can be realized.

The semiconductor device can be provided not only to the door 901201,the glass window 901203, and the ceiling 901204 as shown in FIG. 94, butalso to various places. For example, the semiconductor device can beincorporated with a strap, a seat, a handrail, a floor, and the like. Atthis time, a shape of the display panel 901202 may be changed inaccordance with a shape of an object provided with the semiconductordevice.

FIGS. 95A and 95B show an example where a semiconductor device isincorporated with a passenger airplane.

FIG. 95A shows a shape of a display panel 901302 attached to a ceiling901301 above a seat of the passenger airplane when the display panel901302 is used. The display panel 901302 is incorporated with theceiling 901301 using a hinge portion 901303, and the passenger can viewthe display panel 901302 by stretching of the hinge portion 901303. Thedisplay panel 901302 has a function to display information by anoperation by the passenger and a function to be used as an advertisementor an entertainment means, In addition, when the hinge portion is bentand put in the ceiling 901301 of the airplane as shown in FIG. 95B,safety in taking-off and landing can be assured. Note that when adisplay element in the display panel is lit in an emergency, the displaypanel can also be used as an information transmission means and anevacuation light.

The semiconductor device can be provided not only to the ceiling 901301as shown in FIGS. 95A and 95B, but also to various places. For example,the semiconductor device can be incorporated with a seat, a tableattached to a seat, an armrest, a window, and the like. A large displaypanel which a large number of people can view may be provided at a wallof an airframe. At this time, a shape of the display panel 901302 may bechanged in accordance with a shape of an object provided with thesemiconductor device.

Note that in this embodiment mode, bodies of a train car, a car, and anairplane are shown as a moving object; however, the invention is notlimited thereto, and a semiconductor device can be provided to variousobjects such as a motorcycle, an four-wheel drive car (including a car,a bus, and the like), a train (including a monorail, a railroad car, andthe like), and a vessel. Since a semiconductor device can instantlyswitch images displayed on a display panel in a moving object by anexternal signal, a moving object is provided with the semiconductordevice, so that the moving object can be used as an advertisementdisplay board for an unspecified number of customers, an informationdisplay board in disaster, and the like.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 21

As described above, this specification includes at least the followinginvention.

One aspect of the invention is a liquid crystal display device whichincludes a pixel including a liquid crystal element and a drivercircuit. The driver circuit includes a first transistor, a secondtransistor, a third transistor, a fourth transistor, a fifth transistor,a sixth transistor, a seventh transistor, and an eighth transistor. Notethat the following connection relationships are included in at least apart of the driver circuit. A first electrode of the first transistor iselectrically connected to a fourth wiring, and a second electrode of thefirst transistor is electrically connected to a third wiring. A firstelectrode of the second transistor is electrically connected to a sixthwiring, and a second electrode of the second transistor is electricallyconnected to a third wiring. A first electrode of the third transistoris electrically connected to a fifth wiring, a second electrode of thethird transistor is electrically connected to a gate electrode of thesecond transistor, and a gate electrode of the third transistor iselectrically connected to the fifth wiring. A first electrode of thefourth transistor is electrically connected to the sixth wiring, asecond electrode of the fourth transistor is electrically connected tothe gate electrode of the second transistor, and a gate electrode of thefourth transistor is electrically connected to a gate electrode of thefirst transistor. A first electrode of the fifth transistor iselectrically connected to the fifth wiring, a second electrode of thefifth transistor is electrically connected to the gate electrode of thefirst transistor, and a gate electrode of the fifth transistor iselectrically connected to a first wiring. A first electrode of the sixthtransistor is electrically connected to the sixth wiring, a secondelectrode of the sixth transistor is electrically connected to the gateelectrode of the first transistor, and a gate electrode of the sixthtransistor is electrically connected to the gate electrode of the secondtransistor. A first electrode of the seventh transistor is electricallyconnected to the sixth wiring, a second electrode of the seventhtransistor is electrically connected to the gate electrode of the firsttransistor, and a gate electrode of the seventh transistor iselectrically connected to a second wiring. A first electrode of theeighth transistor is electrically connected to the sixth wiring, asecond electrode of the eighth transistor is electrically connected tothe gate electrode of the second transistor, and a gate electrode of theeighth transistor is electrically connected to the first wiring.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element and the driver circuit, a value of a ratioW/L of the channel width W to the channel length L of the firsttransistor may be the highest among those of W/L of the first to eighthtransistors in the driver circuit.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element and the driver circuit, the value of theratio W/L of the channel width W to the channel length L of the firsttransistor may be twice to five times higher than the value of W/L ofthe fifth transistor in the driver circuit.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element and the driver circuit, the channel length Lof the third transistor may be larger than the channel length L of thefourth transistor.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element and the driver circuit, a capacitor may beprovided between the second electrode and the gate electrode of thefirst transistor.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element and the driver circuit, the first to eighthtransistors may be n-channel transistors.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element and the driver circuit, amorphous silicon maybe used as semiconductor layers of the first to eighth transistors.

Another aspect of the invention is a liquid crystal display device whichincludes a pixel including a liquid crystal element, a first drivercircuit, and a second driver circuit. The following connectionrelationships are included in at least part of the first and seconddriver circuits. The first driver circuit includes a first transistor, asecond transistor, a third transistor, a fourth transistor, a fifthtransistor, a sixth transistor, a seventh transistor, and an eighthtransistor. A first electrode of the first transistor is electricallyconnected to a fourth wiring, and a second electrode of the firsttransistor is electrically connected to a third wiring. A firstelectrode of the second transistor is electrically connected to a sixthwiring, and a second electrode of the second transistor is electricallyconnected to a third wiring. A first electrode of the third transistoris electrically connected to a fifth wiring, a second electrode of thethird transistor is electrically connected to a gate electrode of thesecond transistor, and a gate electrode of the third transistor iselectrically connected to the fifth wiring. A first electrode of thefourth transistor is electrically connected to the sixth wiring, asecond electrode of the fourth transistor is electrically connected tothe gate electrode of the second transistor, and a gate electrode of thefourth transistor is electrically connected to a gate electrode of thefirst transistor. A first electrode of the fifth transistor iselectrically connected to the fifth wiring, a second electrode of thefifth transistor is electrically connected to the gate electrode of thefirst transistor, and a gate electrode of the fifth transistor iselectrically connected to a first wiring. A first electrode of the sixthtransistor is electrically connected to the sixth wiring, a secondelectrode of the sixth transistor is electrically connected to the gateelectrode of the first transistor, and a gate electrode of the sixthtransistor is electrically connected to the gate electrode of the secondtransistor. A first electrode of the seventh transistor is electricallyconnected to the sixth wiring, a second electrode of the seventhtransistor is electrically connected to the gate electrode of the firsttransistor, and a gate electrode of the seventh transistor iselectrically connected to a second wiring. A first electrode of theeighth transistor is electrically connected to the sixth wiring, asecond electrode of the eighth transistor is electrically connected tothe gate electrode of the second transistor, and a gate electrode of theeighth transistor is electrically connected to the first wiring. Inaddition, the second driver circuit includes a ninth transistor, a tenthtransistor, an eleventh transistor, a twelfth transistor, a thirteenthtransistor, a fourteenth transistor, a fifteenth transistor, and asixteenth transistor. A first electrode of the ninth transistor iselectrically connected to a tenth wiring, and a second electrode of theninth transistor is electrically connected to a ninth wiring. A firstelectrode of the tenth transistor is electrically connected to a twelfthwiring, and a second electrode of the tenth transistor is electricallyconnected to the ninth wiring. A first electrode of the eleventhtransistor is electrically connected to an eleventh wiring, a secondelectrode of the eleventh transistor is electrically connected to a gateelectrode of the tenth transistor, and a gate electrode of the eleventhtransistor is electrically connected to the eleventh wiring. A firstelectrode of the twelfth transistor is electrically connected to thetwelfth wiring, a second electrode of the twelfth transistor iselectrically connected to the gate electrode of the tenth transistor,and a gate electrode of the twelfth transistor is electrically connectedto a gate electrode of the ninth transistor. A first electrode of thethirteenth transistor is electrically connected to the eleventh wiring,a second electrode of the thirteenth transistor is electricallyconnected to the gate electrode of the ninth transistor, and a gateelectrode of the thirteenth transistor is electrically connected to aseventh wiring. A first electrode of the fourteenth transistor iselectrically connected to the twelfth wiring, a second electrode of thefourteenth transistor is electrically connected to the gate electrode ofthe ninth transistor, and a gate electrode of the fourteenth transistoris electrically connected to the gate electrode of the tenth transistor.A first electrode of the fifteenth transistor is electrically connectedto the twelfth wiring, a second electrode of the fifteenth transistor iselectrically connected to the gate electrode of the ninth transistor,and a gate electrode of the fifteenth transistor is electricallyconnected to an eighth wiring. A first electrode of the sixteenthtransistor is electrically connected to the twelfth wiring, a secondelectrode of the sixteenth transistor is electrically connected to thegate electrode of the tenth transistor, and a gate electrode of thesixteenth transistor is electrically connected to the seventh wiring.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, the fourth wiring and the tenth wiring may beelectrically connected, the fifth wiring and the eleventh wiring may beelectrically connected, and the sixth wiring and the twelfth wiring maybe electrically connected.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, the fourth wiring and the tenth wiring may be the samewiring, the fifth wiring and the eleventh wiring may be the same wiring,and the sixth wiring and the twelfth wiring may be the same wiring.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, the third wiring and the ninth wiring may beelectrically connected.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, the third wiring and the ninth wiring may be the samewiring.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, a value of the ratio W/L of the channel width W to thechannel length L of the first transistor may be the highest among thoseof W/L of the first to eighth transistors, and a value of the ratio W/Lof the channel width W to the channel length L of the ninth transistormay be the highest among those of W/L of the ninth to sixteenthtransistors.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, the value of the ratio W/L of the channel width W to thechannel length L of the first transistor may be twice to five timeshigher than the value of W/L of the fifth transistor, and the value ofthe ratio W/L of the channel width W to the channel length L of theninth transistor may be twice to five times higher than the value of W/Lof the thirteenth transistor.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, the channel length L of the third transistor may belarger than the channel length L of the fourth transistor, and thechannel length L of the eleventh transistor may be larger than thechannel length L of the twelfth transistor.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, a capacitor may be provided between the second electrodeand the gate electrode of the first transistor, and a capacitor may beprovided between the second electrode and the gate electrode of theninth transistor.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, the first to sixteenth transistors may be n-channeltransistors.

In the liquid crystal display device which includes the pixel includingthe liquid crystal element, the first driver circuit, and the seconddriver circuit, amorphous silicon may be used as semiconductor layers ofthe first to sixteenth transistors.

Various electronic devices can be provided with any of theaforementioned liquid crystal display devices.

Each liquid crystal display device in this embodiment mode correspondsto the liquid crystal display device disclosed in this specification.Therefore, operation effects similar to those in the other embodimentmodes are obtained. This application is based on Japanese PatentApplication serial No. 2006-269905 filed in Japan Patent Office on Sep.29, 2006, the entire contents of which are hereby incorporated byreference.

1. (canceled)
 2. A semiconductor device comprising: a first transistor,a second transistor, a third transistor, a fourth transistor, a fifthtransistor, a sixth transistor, a seventh transistor, and an eighthtransistor; a first wiring, a second wiring, a third wiring, a fourthwiring, a fifth wiring, a sixth wiring, and a seventh wiring, whereinone of a source and a drain of the first transistor is electricallyconnected to the fourth wiring, and the other of the source and thedrain of the first transistor is electrically connected to the thirdwiring, wherein one of a source and a drain of the second transistor iselectrically connected to the sixth wiring, and the other of the sourceand the drain of the second transistor is electrically connected to thethird wiring, wherein one of a source and a drain of the thirdtransistor is electrically connected to the seventh wiring, the other ofthe source and the drain of the third transistor is electricallyconnected to a gate of the second transistor, and a gate of the thirdtransistor is electrically connected to the seventh wiring, wherein oneof a source and a drain of the fourth transistor is electricallyconnected to the sixth wiring, the other of the source and the drain ofthe fourth transistor is electrically connected to the gate of thesecond transistor, and a gate of the fourth transistor is electricallyconnected to a gate of the first transistor, wherein one of a source anda drain of the fifth transistor is electrically connected to the fifthwiring, the other of the source and the drain of the fifth transistor iselectrically connected to the gate of the first transistor, and a gateof the fifth transistor is electrically connected to the first wiringthrough at least a first conductive layer, wherein one of a source and adrain of the sixth transistor is electrically connected to the sixthwiring, the other of the source and the drain of the sixth transistor iselectrically connected to the gate of the first transistor, and a gateof the sixth transistor is electrically connected to the gate of thesecond transistor, wherein one of a source and a drain of the seventhtransistor is electrically connected to the sixth wiring, the other ofthe source and the drain of the seventh transistor is electricallyconnected to the gate of the first transistor, and a gate of the seventhtransistor is electrically connected to a second wiring through at leasta second conductive layer, wherein one of a source and a drain of theeighth transistor is electrically connected to the sixth wiring, theother of the source and the drain of the eighth transistor iselectrically connected to the gate of the second transistor, and a gateof the eighth transistor is electrically connected to the first wiring,wherein the first conductive layer and the second conductive layer havea transparency, and wherein the first conductive layer has a first widthand the second conductive layer has a second width narrower than thefirst width.
 3. The semiconductor device according to claim 2, wherein avalue of a ratio W/L of a channel width W to a channel length L of theseventh transistor is 1/40 to 1/10 than that of the first transistor. 4.The semiconductor device according to claim 2, wherein a value of aratio W/L of a channel width W to a channel length L of the firsttransistor is the highest among values of W/L of the first to eighthtransistors.
 5. A display device comprising a pixel and thesemiconductor device according to claim 2, wherein the pixel iselectrically connected to the third wiring.
 6. A display modulecomprising a Flexible Printed Circuit (FPC) and a display deviceaccording to claim
 2. 7. An electronic device comprising the displaymodule according to claim 2, wherein the electronic device is oneselected from the group consisting of a camera, a computer, an imagereproducing device, a goggle-type display, and a game machine.
 8. Asemiconductor device comprising: a first transistor, a secondtransistor, a third transistor, a fourth transistor, a fifth transistor,a sixth transistor, a seventh transistor, and an eighth transistor; afirst wiring, a second wiring, a third wiring, a fourth wiring, a fifthwiring, a sixth wiring, and a seventh wiring, wherein one of a sourceand a drain of the first transistor is electrically connected to thefourth wiring through at least a first conductive layer, and the otherof the source and the drain of the first transistor is electricallyconnected to the third wiring, wherein one of a source and a drain ofthe second transistor is electrically connected to the sixth wiring, andthe other of the source and the drain of the second transistor iselectrically connected to the third wiring, wherein one of a source anda drain of the third transistor is electrically connected to the seventhwiring, the other of the source and the drain of the third transistor iselectrically connected to a gate of the second transistor, and a gate ofthe third transistor is electrically connected to the seventh wiring,wherein one of a source and a drain of the fourth transistor iselectrically connected to the sixth wiring, the other of the source andthe drain of the fourth transistor is electrically connected to the gateof the second transistor, and a gate of the fourth transistor iselectrically connected to a gate of the first transistor, wherein one ofa source and a drain of the fifth transistor is electrically connectedto the fifth wiring, the other of the source and the drain of the fifthtransistor is electrically connected to the gate of the firsttransistor, and a gate of the fifth transistor is electrically connectedto the first wiring, wherein one of a source and a drain of the sixthtransistor is electrically connected to the sixth wiring, the other ofthe source and the drain of the sixth transistor is electricallyconnected to the gate of the first transistor, and a gate of the sixthtransistor is electrically connected to the gate of the secondtransistor, wherein one of a source and a drain of the seventhtransistor is electrically connected to the sixth wiring, the other ofthe source and the drain of the seventh transistor is electricallyconnected to the gate of the first transistor, and a gate of the seventhtransistor is electrically connected to a second wiring through at leasta second conductive layer, wherein one of a source and a drain of theeighth transistor is electrically connected to the sixth wiring, theother of the source and the drain of the eighth transistor iselectrically connected to the gate of the second transistor, and a gateof the eighth transistor is electrically connected to the first wiring,wherein the first conductive layer and the second conductive layer havea transparency, and wherein the first conductive layer has a first widthand the second conductive layer has a second width narrower than thefirst width.
 9. The semiconductor device according to claim 8, wherein avalue of a ratio W/L of a channel width W to a channel length L of theseventh transistor is 1/40 to 1/10 than that of the first transistor.10. The semiconductor device according to claim 8, wherein a value of aratio W/L of a channel width W to a channel length L of the firsttransistor is the highest among values of W/L of the first to eighthtransistors.
 11. A display device comprising a pixel and thesemiconductor device according to claim 8, wherein the pixel iselectrically connected to the third wiring.
 12. A display modulecomprising a Flexible Printed Circuit (FPC) and a display deviceaccording to claim
 8. 13. An electronic device comprising the displaymodule according to claim 8, wherein the electronic device is oneselected from the group consisting of a camera, a computer, an imagereproducing device, a goggle-type display, and a game machine.
 14. Asemiconductor device comprising: a first transistor, a secondtransistor, a third transistor, a fourth transistor, a fifth transistor,a sixth transistor, a seventh transistor, and an eighth transistor; afirst wiring, a second wiring, a third wiring, a fourth wiring, a fifthwiring, a sixth wiring, and a seventh wiring, wherein one of a sourceand a drain of the first transistor is electrically connected to thefourth wiring, and the other of the source and the drain of the firsttransistor is electrically connected to the third wiring through atleast a first conductive layer, wherein one of a source and a drain ofthe second transistor is electrically connected to the sixth wiring, andthe other of the source and the drain of the second transistor iselectrically connected to the third wiring, wherein one of a source anda drain of the third transistor is electrically connected to the seventhwiring, the other of the source and the drain of the third transistor iselectrically connected to a gate of the second transistor, and a gate ofthe third transistor is electrically connected to the seventh wiring,wherein one of a source and a drain of the fourth transistor iselectrically connected to the sixth wiring, the other of the source andthe drain of the fourth transistor is electrically connected to the gateof the second transistor, and a gate of the fourth transistor iselectrically connected to a gate of the first transistor, wherein one ofa source and a drain of the fifth transistor is electrically connectedto the fifth wiring, the other of the source and the drain of the fifthtransistor is electrically connected to the gate of the firsttransistor, and a gate of the fifth transistor is electrically connectedto the first wiring, wherein one of a source and a drain of the sixthtransistor is electrically connected to the sixth wiring, the other ofthe source and the drain of the sixth transistor is electricallyconnected to the gate of the first transistor, and a gate of the sixthtransistor is electrically connected to the gate of the secondtransistor, wherein one of a source and a drain of the seventhtransistor is electrically connected to the sixth wiring, the other ofthe source and the drain of the seventh transistor is electricallyconnected to the gate of the first transistor, and a gate of the seventhtransistor is electrically connected to a second wiring through at leasta second conductive layer, wherein one of a source and a drain of theeighth transistor is electrically connected to the sixth wiring, theother of the source and the drain of the eighth transistor iselectrically connected to the gate of the second transistor, and a gateof the eighth transistor is electrically connected to the first wiring,wherein the first conductive layer and the second conductive layer havea transparency, and wherein the first conductive layer has a first widthand the second conductive layer has a second width narrower than thefirst width.
 15. The semiconductor device according to claim 14, whereina value of a ratio W/L of a channel width W to a channel length L of theseventh transistor is 1/40 to 1/10 than that of the first transistor.16. The semiconductor device according to claim 14, wherein a value of aratio W/L of a channel width W to a channel length L of the firsttransistor is the highest among values of W/L of the first to eighthtransistors.
 17. A display device comprising a pixel and thesemiconductor device according to claim 14, wherein the pixel iselectrically connected to the third wiring.
 18. A display modulecomprising a Flexible Printed Circuit (FPC) and a display deviceaccording to claim
 14. 19. An electronic device comprising the displaymodule according to claim 14, wherein the electronic device is oneselected from the group consisting of a camera, a computer, an imagereproducing device, a goggle-type display, and a game machine.